It seems hard to remember when America was united. But, recently, we were reminded of such a time--the morning of September 11th, 2001 -- when all Americans pledged to persevere, together. Nearly a quarter century ago, the New York City Office of the Chief Medical Examiner made a promise to identify the remains of the lost souls of 9/11. Not many are aware but that work has never stopped. Today, more than 1,000 families still wait for word. And you're about to meet two for whom the promise was kept. The most recent identification came this past December when Ellen Niven was decorating her Christmas tree. Two police officers came to her door with news of her husband, John, who had been missing 22 years.

Ellen Niven: John was my husband, I met him when I was 24 years old and had moved to New York. Incredible person, described by people who knew him as a real gentleman, very old school, old soul. Wonderful father. Very happy when we had our young son and spent a lot of time with him. Great friend to a lot of-- a lot of people.

John Niven was a 44-year-old insurance executive bound for his office on the 105th floor of the South Tower as the terrorist attack began.

Scott Pelley: The first building was hit; John was in the second building.

Ellen Niven: The South Tower, yes.

Scott Pelley: And he had an opportunity to call you.

Ellen Niven: Yes, he said, "Hi, honey, it's me. If you hear anything on the news, don't worry, I'm okay. It was the other building."

In the "other building," a different family tragedy was unfolding. Twenty-five-year-old, "Haberman, Andrea L." had just received a visitor pass on her first trip to her company's headquarters on the 92nd floor. Back home in Chicago, "Andy" as her family called her, had just been fitted for her wedding dress. In Wisconsin, her mother Kathy, was watching the news.

Kathy Haberman: I was shocked. And I ran upstairs to wake up Gordy, to tell him. And then, I came back downstairs just in time to see the second plane hit.

That was United Flight 175, as Gordon Haberman joined his wife.

Gordon Haberman: I threw a cup against the wall. I remember that. Thing is, we didn't know what tower she was in. We didn't know where she was.

The search for Andrea Haberman, John Niven and nearly 3,000 others would become the passion of Dr. Charles Hirsch, the city's chief medical examiner. He raced to the base of the burning towers with a team that included a young scientist named Mark Desire.

Mark Desire: Both towers were standing. They were on fire. We parked our truck. We were to set up a temporary morgue and begin to preserve the evidence. Wasn't down there very long. I had just received my orders from Dr. Hirsch. I picked up our gear box and the South Tower cracked. It was-- it was right over us, that plume. I could see the steel and the fire coming down. And I thought, "This is it. This is how I die."

The South Tower, with John Niven inside, foundered after 56 minutes.

Scott Pelley: As you're running away from the collapsing South Tower, you were heading for a door in an adjacent building. And then, you got blasted off your feet?

Mark Desire: Yeah, it just knocked me right out of my shoes. Never made it to the door. But it was enough to get me through-- through the-- partially through the window, which really what saved my life. If I was on the outside, everything that came down across my legs would have-- would-- taken me out.

The medical examiner's team survived. That's Mark Desire, in the middle in the green shirt. The North Tower, in the distance, is minutes from collapse. Andrea Haberman is inside. Her parents, her sister Julie and fiancé, Al, drove 16 hours to Manhattan where they picked up a list of hospitals.

Gordon Haberman: But with those lists of medical centers-- Kathy and I split up -- and Julie and Al took the west side of Broadway. And we took the east side. 32 different medical centers, working our way down towards the tip of Manhattan and Ground Zero. 'Course, the answers were--

"No" for Andrea Haberman and, thousands of others. Manhattan was papered in pleas for the missing. and longing remained after hope had washed away.

Julie Osmus: Everybody has a flier. Everybody is looking for their people...

Families lined up at a National Guard armory and waited hours to give DNA samples to the medical examiner.

Mark Desire: 17,000-- 17,000 reference samples; toothbrushes, razors, hairbrushes, anything that the person touched when they were alive. If we couldn't get one of those samples, what living relatives do we have? Moms and dads, kids...

Ellen Niven: There was DNA swab done of my young son, Jack's cheek. You filled out descriptions, you gave photographs. You filled out many, many forms.

Scott Pelley: They swabbed your son's cheek for DNA.

His father was among those entombed in a mountain of misery. Nearly two million tons of debris were searched by hand for human remains. After a year, they thought they had found everything, but then in 2006, there was a shocking revelation-- bone fragments on the roof of a building across the street from ground zero. The medical examiner sent anthropologist Bradley Adams.

Bradley Adams: We ended up going through the whole rooftop-- and we found over 700 small bone fragments on that rooftop. And then we ended up, you know, obviously if there's remains there, we need to search other areas. So, we went through every floor of that building, even to the point of having vacuum cleaners and vacuuming up dust and debris.

The remains on the Deutsche Bank building were from American Airlines Flight 11. The discovery prompted a new search for clues at ground zero.

Bradley Adams: Computer floppy disks or golf balls or parts of office furniture that would be buried there. And if you're seeing that, then you know there's the potential there could be human remains mixed in with this World Trade Center debris.

Five years after the attack, Brad Adams began collecting 18,000 tons of excavation material over the course of a year. Seventy five anthropologists washed it through screens.

Scott Pelley: How many human remains did you find in that project?

Bradley Adams: There was the 700 on Deutsche Bank, and then over a thousand more were found during the sifting operations.

All together, the total World Trade Center remains came to 21,905.

Dr. Jason Graham: The recovery efforts have been monumental, and this was an unprecedented event as you know, this is the greatest mass murder in the history of the United States.

Today, Dr. Jason Graham is New York City's chief medical examiner. He inherited this promise made by his late predecessor, Charles Hirsch.

Dr. Jason Graham: As long as there are families who are continuing to seek answers, this work will continue.

Scott Pelley: What's the scope of what's left to be done?

Dr. Jason Graham: There were 2,753-- victims-- homicide victims. 60 percent of those individuals have been identified. 40 percent-- are left to be identified.

Forty percent comes to 1,103 victims with no identified remains.

Mark Desire: So, these are the steps from once remains are received...

Putting a name to those remains is the job of the last original member of the medical examiner's 9/11 team, Mark Desire, now assistant director of forensic biology.

Mark Desire: These remains went through every possible thing that could destroy DNA, from jet fuel to diesel fuel, mold, bacteria, sunlight, all kinds of chemicals that were in the building, insects, heat, fire. All these things destroy DNA. Everything was present at Ground Zero, making this not only the largest forensic investigation in the history of the United States, but the most difficult.

Scott Pelley: Some of these World Trade Center remains have been tested how many times?

Mark Desire: Without a result. But if there's DNA, we're gonna find it. We're gonna find it. We're gonna generate a profile. It may take us a while.

All remains today are bone. In a demonstration with animal bone, Desire showed us new technologies that make breakthroughs possible. They include this cryogenic grinder, filled with liquid nitrogen at 320 degrees below zero.

Mark Desire: The early days of 9/11, 2001, we were doing this all by hand with a mortar and pestle.

With high-speed vibration, individual cells in the deeply frozen bone shatter -- a chemical process releases their DNA.

Mark Desire: Equipment like this has taken it to the next level, given us so much more access to cells. We need as much DNA as possible because these samples have hardly any.

Other innovations chemically amplify DNA revealing more information from the smallest fragment.

Mark Desire: Some as small as the size of a Tic Tac, we've been able to get DNA from those and generate a DNA profile.

Samples are tested every week with advanced technology. John Niven's bone fragments, 15 in all, had been tested for years. Then, last year, the lab made a "perfect" match to the swab of the cheek of his infant son -- taken 22 years before. First notifications are made in person.

Ellen Niven: And the police came to the door, and my first reaction was, I said, "Is-- is it my son?" And they said, "No, everything's okay." And these two wonderful, really kind-- policemen said-- "We're here to deliver you the news," and they had a letter, "That your husband's DNA has been discovered.

Scott Pelley: When the police officers said, "we've found your husband's DNA," I mean, that must've hit you as quite a shock.

Ellen Niven: It was a shock that they've been looking all these, all these 22 years. I thought that that door had long been closed.

About half of 9/11 families have told the medical examiner that if their loved one is identified today-they don't want to know. Time has lightened their burden of grief. But the other half still hope for word. Few understand this mix of emotions like Dr. Jennifer Odien. She's the medical examiner's World Trade Center anthropologist -- a scientist and something of a counselor to those still hoping for the promise.

Dr. Jennifer Odien: Shock, I would say is the first response, typically, just because of how many years have gone by. They weren't necessarily expecting to get that...that new identification. Any identification for some families they... they weren't expecting it. And then after that, it tends to be emotional, you know, some grief. Because now all the memories, everything, is coming up about that.

Scott Pelley: Can you give me a sense of how many families you're in touch with?

Scott Pelley: What memories have some of these families shared with you?

Dr. Jennifer Odien: Some of the memories are that last phone call that they received. Or the last birthday they had or, you know, a vacation. But a lot of it has to do with that last contact, whether it was in the morning before they went off to work or if they had called while they were in the towers.

Scott Pelley: A vital part of your job is to listen.

Dr. Jennifer Odien: Yes. And I will listen as long as they would like me to. We have phone calls sometimes, and it'll last an hour. And I will stay on and listen and talk-- to them when they have a question, I'll answer it. But a lotta times they just want to speak to someone.

Ellen Niven: I talked to Dr. Odien, Jen, who was wonderful and so kind and so nice.

Ellen Niven spoke to Jennifer Odien this past December when the remains of her husband, John, were identified for the first time.

Ellen Niven: So, I heard nothing about John's remains for 22 years. So, we just assumed that there was nothing. We buried a box of mementos, photographs and a letter, that I wrote, and a drawing my son had done. And then nothing.

She remarried and had two more boys. Her son, Jack, was 18 months old when his father died. Now age 24, Jack let his mother tell the story of how his father's identification struck them -- differently.

Ellen Niven: For me, it was very sad, for him, it was uplifting, in a way, to realize that people had been working all that time to find any piece of his dad. And that of all the people that were blessed by this breakthrough, that it was his dad. You know, that meant so much to him. So, it was really moving to see how moved he was. So many people who had met John, or had even not met John, reached out to us. Emails, letters, phone calls to me and to Jack. And I think that for Jack, it really brought to life so many descriptions of his father that, as a young man he could now really appreciate. So it was a great remembrance. John being able to be back in a lot of people's minds.

Dr. Jennifer Odien: As remains are identified, the folders get bigger and bigger because we keep adding all the information for those remains.

Jennifer Odien adds new identifications to the "DM" files. No one knew what to call 9/11 the day it happened so the M.E. settled on Disaster Manhattan. There's one folder for each murder victim -- 2,753.

Scott Pelley: This is an inventory of all of the remains found for this one person.

Dr. Jennifer Odien: Correct.

Scott Pelley: Such things here as rib, vertebrae, sternum, found over months and years.

Dr. Jennifer Odien: Correct.

Scott Pelley: This indicates that about 50 percent of this body was recovered.

Dr. Jennifer Odien: Roughly. Yes. And it's a very rough estimate. We're trying to understand how many of the remains have been identified, how much of that individual is there, what are the chances that more remains will be identified? 'Cause those are questions that families will ask.

Families have a choice. They can ask a funeral home to pick up a remain, vacuum packed like this, labeled with an American flag. Or they can leave the remain in the custody of the medical examiner.

Dr. Jennifer Odien: I tell them that they don't have to make that decision right now. They can call back in a month, a year, two years, ten years. And we could then have those remains transferred over to the funeral home that they choose.

Scott Pelley: You are keeping track of the remains that precisely.

Dr. Jennifer Odien: Oh absolutely. We know where every single remain is.

Scott Pelley: How do you do that?

Dr. Jennifer Odien: We have numbers associated with all remains. So, every remain that was recovered has a specific unique number associated with it.

Number 18,756 is the most recent remain of Andrea Haberman.

The Habermans have asked to be told of all new identifications. For them, each reminder of their daughter is a steppingstone through a void... that Gordon Haberman calls "missing."

Gordon Haberman: It's hard to describe "missing" to other people, but it's deep inside you. There isn't a day that I or we don't think of her.

Scott Pelley: Help me understand what it means to you to have had Andrea's remains identified?

Gordon Haberman: If Andrea could face what she had to face, how could I not wanna know what happened to her?

Today he's 73. His relationship with the medical examiner has spanned 11 notifications -- plus--the amazing discovery of the relics of Andrea.

Scott Pelley: When you went through these things for the first time, what did you see, what did you think?

Gordon Haberman: How terrible it musta been.

We met Gordon Haberman at the national September 11th Memorial Museum at ground zero. With the help of the museum staff, we saw artifacts from Andrea's purse which are archived, cataloged and handled like antiquities. He received them in 2004 from the NYPD in a meeting with officers and a priest.

Gordon Haberman: They wanted to know if I needed any, any help processing that. And I was actually more concerned at that time how I'm gonna keep these from my wife.

He feared his wife's pain, so he locked the bag in a desk drawer which he did not open for seven years. In 2011 they donated to the museum the collection of a quarter century ago.

Gordon Haberman: This is the phone that we kept calling.

A pager...driver's license and the last photo of her life, the visitor I.D. that captured Andrea's spirit minutes before she was gone.

Gordon Haberman: And that was our Andrea. And she was gonna go on to do great things and she wanted grandchildren, and her house was such a pride and she loved her Al so much...

He brought Andrea's identified remains home to Wisconsin but he believes her other remains, still unidentified, are in the museum behind this wall and a verse by the poet Virgil; "no day shall erase you from the memory of time." Many museum visitors don't realize, but this is the outer wall of the medical examiner's repository for 9/11 remains.

Scott Pelley: We're in what was the basement of the North Tower...

Gordon Haberman: Yes.

Scott Pelley: and this is completely out of the path of the museum it's hidden around the corner...

Gordon Haberman: Yes.

Next to the repository, this is the entrance to the Office of the Chief Medical Examiner family reflection room. The room and repository have never been seen by the public. Families, only, can call a number on the door which summons an escort, often, Dr. Jennifer Odien:.

Dr. Jennifer Odien: So, the visits are different every time. Some families are very emotional, and I'll sit in with them for an hour and just hear stories. And I'll walk around with them until I know that they're okay and then I leave the room completely, so they have the space to themselves.

Scott Pelley: This must be a burden to you.

Dr. Jennifer Odien: I don't consider it a burden. It's tough. I definitely have moments of, you know, feeling very emotional and needing to step back. But when I talk to a family and they say thank you, how grateful they are with our continued work, that a question I've answered helped them in some way, it makes it all worth it.

Gordon Haberman invited us inside as his guest. No camera, but we were allowed to record the audio. We found a small sitting room and a window into the repository for human remains.

Scott Pelley: The window we're looking through looks like it's about five feet wide and three feet or so tall. Just a single window, and a single wooden bench in front of the window.

With permission, we gave our notes to an artist who sketched the view through the window that joins the family room to the repository. A loved one, sitting on the bench, sees a deep, austere, white room with rows of dark wooden cabinets eight feet tall. They hold about 10,000 remains -- both known and unknown. It is, in a sense, a private national shrine.

Scott Pelley: Why do you come here after all these years?

Gordon Haberman: I feel close to my daughter. She wasn't meant to be here, but she's here.

The repository, in the museum which stands between the reflecting pools, seems like the right place, Ellen Niven told us, for her husband, John.

She has visited the pools over the decades to run her fingertips over her lost husband's tribute. And yet she is surprised how the endless effort to actually find him allowed her to feel, once again, the embrace of a nation's devotion.

Ellen Niven: My first reaction was to tell people, "Did you know that all this time they have been sifting through these remains, and researching and researching for-- for over 20 years?" What an incredible thing. You know, John had another moment in-- in all of our lives. So that was something I'm incredibly grateful for.