Transcript for Ex-Port Authority detective says he warned about terrorism threat a decade before 9/11
to stop it. Former port authority detective Matthew baa Shaer is heading down New York City's west side highway on a trip he swore he would never make. It's not going to be easy. Reporter: The skyline now dominated by the freedom tower, not bashear's beloved twin towers. They're my buildings. I was port authority. I thought they were just stunningly beautiful. Reporter: Bashear has not been to Manhattan since he helped at ground zero in 2001. The last time I was here this was nothing more than 15 stories of rubble. Reporter: He's here confronting his darkest demons, sharing his story for the first time on network television. It doesn't get any easier. It's 16 years, and it doesn't get any easier. Because this should never have happened. This should never have happened. Reporter: His story one of the many pieces of the September 11th puzzle put together in history's new documentary "Road to 9/11." I was hunting the guy that did this. And I didn't get him. My life has been forever changed. Reporter: For bashear the road to 9/11 started in the 1970s when the world trade center was being built. As a child I would take my allowance and I would jump on the lirr local and I would watch them blasting and digging a whole for the foundation of the trade center. Sow really had a connection to these buildings. Yeah. I watched them being built, I worked there, and I sifted through the rubble for my 37 guys that I lost that day. Reporter: His losses on September 11th were deeply personal. Working in the port authority's intelligence unit, he says he knew a decade earlier that the world trade center would be a terrorist target. 1991 leading into 1992 I was asked by the commanding officer of the world trade center to do a vulnerability study. My report that I submitted was that the building was vulnerable to attack in the parking garages. Reporter: Were your warnings heeded? You complete the report and you pass it up the chain. What they do with it after that is their business, not mine. Repurter: But just a year after basheer's report -- Watch out, watch out. One, two, three. Reporter: More than 1,000 people were hurt and six killed when a bomb went off inside a van parked at the world trade center garage. It was an eye opener at that time. But of course as we get further down the road and other things have happened, it was really a huge warning sign. Reporter: Basheer would then team up with the FBI, eventually helping track down and capture the mastermind of the 1993 attack, ramzi Yusuf, who gave law enforcement his own grim prediction about the twin towers the night they took him into custody in New York. It was February 7th, 1995, about 9:30 at night when we came around lower Manhattan. And we took the blindfold off of him and said see, ramzi, they're still standing. And he took the time to look us each in the eye in the helicopter. And he said, "Next time we'll have more money and we will bring them down." Reporter: It would be Yusuf's uncle, Khalid shaikh Mohammed, or ksm, who would fulfill that promise six years later, working at the behest of Al Qaeda leader Osama bin laden. We knew in the mid 1990s that Khalid shaikh Mohammed was trouble. Reporter: Richard Clarke was the chief counterterrorism adviser on the national security council at the time, overseeing the growing threat from Al Qaeda operatives like ksm. We were looking for him. And we had an indictment, a sealed indictment against him from a federal grand jury. Reporter: He was wanted for his role in planning an attack on the pope and an elaborate plot to bring down planes over the pacific ocean. Both of those plots were foiled. To many of the people that knew him he was the boss. So when you're looking at somebody who's thought of that way you've got to put more effort into him. Reporter: Basheer along with his partner frank Pellegrino crisscrossed the globe in the 1990s hunting terrorists including ksm. The U.S. Government tracked him to Qatar in 1996 but Clarke says when he requested help in capturing ksm he was turned down. We asked the FBI. We asked the CIA. We asked the Pentagon. Can any of you mount a snatch operation to go into Doha and pick him up and fly him out? And all of them said that was not within their capacity. Reporter: Ksm would go on to hand-pick the 19 terrorists who hijacked the planes on September 11th. But trying to stop terrorists before they acted was not the only frustration in the years leading up to 9/11. Since there had never been a major foreign terrorist attack, a catastrophic attack, in the U.S., even though the evidence suggested it could happen, a lot of people didn't believe, truly believe that it would happen. Oh, my god! We just got a report in that there's been some sort of explosion at the world trade center in New York City. Reporter: Basheer had retired from the port authority in 2000. But he says when the first plane hit that morning in 2001 he knew exactly what was happening and who was responsible. And I felt my entire insides just fall out. And as I was standing there watching it I saw the second plane come in and hit. And I knew they came back. At that point I lost it. I just sank to the floor and I just started bawling. And as I was doing that, my phone rang. And it was my partner. He was crying too. And he said, bash, look what they did to us. Reporter: Basheer says he grabbed his gear, got in his car, and drove through the night to ground zero. What did you do? Initially, I didn't know what to do. I just -- I just knew I needed to do something. Reporter: And you were allowed to do this kind of work even though you were already retired? Yes. The people from the port authority knew I was back. Actually, they told one of the range officers to sign out my old weapon and give it to me. Reporter: He stayed for two weeks. When I drove out of Manhattan that day, I've never been back. Reporter: Why not? It's too painful a memory for me. I live with a lot of guilt, that I let those guys down. That I lost a lot of good friends because I didn't get it done. Reporter: Khalid shaikh Mohammed would finally be captured in Pakistan on March Amo bay. For Matthew basheer the lists of names at the 9/11 memorial are filled with stories he does not want the world to forget. Bruce Reynolds. There wasn't a Kinder, nicer guy than Bruce Reynolds. I need to keep their memory alive. That's what's important for me. But they still did what they needed to do. They still needed to be god's warriors to get those people out of that building. But I never wanted to see their names etched like this. And "Road to 9/11" is currently airing on history, which is owned in part by our parent company, Disney.
This transcript has been automatically generated and may not be 100% accurate.