EXCLUSIVE: How Washington gunman was left traumatized after being at the ENTRANCE to World Trade Center when South Tower collapsed on 9/11. His stepfather reveals all in first interview Washington Navy Yard gunman Aaron Alexis was present at the World Trade Center on 9/11 He was exiting the subway as the first tower collapsed and joined first responders desperately trying to save people Traumatized from pulling bodies from the mangled wreckage - he didn't sleep for days afterwards Step dad Frank Calderon, who lived with Alexis at the time, said he didn't see any initial signs of PTSD Aaron's birth father Algernon had told police in 2004 that his son was suffering from PTSD as a result of witnessing the events at ground zero By Paul Thompson In Virginia The full horror of the 9/11 terror attack witnessed by Navy Yard gunman Aaron Alexis that could have caused his PTSD has been revealed for the first time by his stepfather. In the first interview by a member of the killer's family, Frank Calderon, 47, said his stepson was going up the escalator from the subway station below the World Trade Center when the first of the towers collapsed. Alexis joined first responders in desperately trying to save people trapped by the collapsing building in downtown New York City. Aaron Alexis, 34, who killed 12 people at the Washington Navy Yard on Monday was present at the World Trade Center on Sept 11 2001 and helped to pull people from the wreckage He was left so traumatized from pulling bodies from the mangled wreckage that he did not sleep for up to three days afterwards. The 12th anniversary of the 9/11 terror attack took place a less than a week ago with memorial services at Ground Zero and also at the Pentagon in the Washington DC. Family members said he suffered from Post-Traumatic Stress Syndrome and blamed the illness for two violent gun related incidents. Investigators are looking to PTSD as a cause for the paranoia and delusions that led him to seek medical help just weeks before going in his deadly rampage killing 12 people. Frank Calderon sits in his car outside his home near Richmond, Va., as he spoke to Mail Online about his former step-son Aaron Alexis Alexis called police to his hotel in Rhode Island where he said he was being followed and that he heard voices in his head. The 34-year-old's mental health problems are thought to be the main cause for the rampage at the Washington Navy Yard where 12 people were shot dead. In an exclusive interview with Mail Online, Mr Calderon fought back tears as he said he is struggling to understand the "loving boy" he raised as his own could have become a mass killer. He is the only family member to openly speak about Alexis as investigators try to find a motive for the killing spree. "What happened in Washington was not from the Aaron I knew and loved," he said. Growing up he was a loving boy. He was a kind boy. There was not violence. In all the years I was with him there was no hint of any violence. Smoke rising from the burning twin towers of the World Trade Center on Sept 11, 2001: Washington Navy Yard gunman Aaron Alexis saw the first tower fall as he emerged from the subway that day Frank said Aaron was working in the IT department in a building adjacent to the World Trade Center when terrorists launched their murderous attack on September 11 2001. Mr Calderon said his stepson was coming up the escalator from the subway station when the first of the towers collapsed. He said: "He was on his way to his shift. The planes had hit the towers, but Aaron said he was on the escalator coming up from the subway station when he saw the building come down. It was right in front of him, and he said he saw it collapse. That day was traumatic for everyone involved. He did not talk much about it, but he said he helped out. He was a strong young man so he would have done what he could. I also worked at the site, but in another building. My shift had ended at 7am and I was already home by the time the planes hit. The South Tower of the World Trade Center was the first to collapse shortly before 10am on September 11. Frantic Search: Emergency responders arrive at the scene of a shooting at the Washington Navy Yard in Washington, where several people were killed and others injured when at least one gunman opened fire at the U.S. Navy Yard One of Alexis' victims was Richard Michael Ridgell and a candlelight vigil was held in his honor at Jaycee Park in Westminster, Md. on Tuesday The North Tower collapsed 29 minutes later causing most of the casualties among the fire crews who had been trying to get people out of the burning building. Aaron and his stepfather were living in Queens, New York, at the time of the terror attack. He said Aaron was a proud New Yorker and like many people he was affected by the events of that day. But Frank, who became step dad to Aaron when he was eight years old, said he did not see any initial signs of PTSD. It was a traumatic time, not just for us but all the people in New York. I can't believe that what happened back then was to blame for what went on this week. One theory that will be probed by investigators in Washington is if traumatic memories from 12 years ago played any part in triggering Monday's shooting rampage. Aaron's birth father Algernon had told police in 2004 that his son was suffering from PTSD as a result of witnessing the events at Ground Zero. He made his statement to police after his son was arrested in Seattle for shooting out the tires of a construction worker. Aaron told police he had an "anger fueled" black out and didn't remember the incident until an hour afterwards. Vigil for the victims: Frank Calderon, who became step dad to Aaron when he was eight years old, said he did not see any initial signs of PTSD in Alexis after 9/11 Chaos: More than 3,000 people work at the site in Washington D.C. where gunfire erupted on Monday He was later involved in another shooting incident while living in Fort Worth, Texas, when a neighbor complained he had fired a bullet into her floor. Aaron, a former Navy aviation technician, did not face prosecution over either incident. His clean record enabled him to buy a shotgun used in the Navy Yard rampage and also get security clearance as a civilian contractor working in the sprawling Navy Yard office complex. Mr Calderon said he had not spoken to his stepson since 2007. The last time we spoke was when he went off to the Navy. He was so excited to be joining the military. I was very close to Aaron and we had talked about me also joining the military, but I was too old. I know he was really looking forward to it. He was at my house in North Carolina before he left for boot camp. You could see the excitement in his face. Frank said he was part of Aaron's life from the age of eight until he was in his mid-20s. John Roger Johnson, left, and Kathy Gaarde, right, were two of Alexis' 12 victims on Monday A moment of silence was held during a candlelight vigil at Jaycee Park in Westminster, Md., on Tuesday in honor of Richard Michael Ridgell who was killed in Monday's shooting at the Navy Yard in Washington He was married to Aaron's mother Cathleen for two years but lived with her for a number years before their wedding. After their split he stayed in contact with the family, in particular Aaron, saying he had an "on-off" relationship with his mother. Sitting in his car outside his home in Virginia, Mr Calderon struggled to comprehend the grief and outrage caused by his son. Staring down at the floor he had a vacant look on his face as he remembered a "loving and kind boy" who loved to play chess. "What took place the other day is not something that I can ever understand," he said. I wish I had the words to explain, but I don't. My thought are not with Aaron but for the victims. My prayers are with their families and I just want to say how sorry I am. Why would he do this? I have not seen or spoken to Aaron since 2007 but I still can't believe that he would do such a thing. Escape: Employees at the naval yard run from the building with their hands in the air following the shooting Traumatized from pulling bodies from the mangled wreckage on 9/11, Alexis didn't sleep for days afterwards and later developed Post-Traumatic Stress Syndrome - possibly as a result of what he saw at ground zero He was not a violent boy. The Aaron I knew loved to learn. We would watch the Discovery Channel together. He was like a sponge and wanted to learn so much. In all the years I was with him I never once witnessed any violence. I was his step-father but because I was not that much older than him I was more like a big brother. We always got on well and he was never any trouble. I just wish I knew what happened to him. Something changed him because the Aaron I knew was not violent. Mr Calderon said his stepson's years in the Navy before his discharge in 2011 must have changed him. I am hearing all these reports about mental illness, that he had paranoia and heard voices. If that is true he should have had help," he said. Military veterans should be given the red carpet treatment. They should get all the healthcare they need and be helped to get jobs. What kind of country are we that a veteran comes back and he has to work in McDonalds. That is not treating them right. Vishnu Pandit, left, and Mary Francis Knight, right, were victims of Monday's massacre Salute: Defense Secretary Chuck Hagel, right, leads a delegation at the Navy Memorial in Washington to remember the victims of Monday's deadly shooting at the Washington Navy Yard on Tuesday Mr Calderon said Aaron had two relatives who lived in Thailand and would often visit them during his breaks from the military. "He did love Thailand and its people, but when he went there it was to see his uncle and cousins," he said. Mr Calderon, a bank worker, said he was visited by two FBI agents in the hours after the shooting. "There was not much to tell them," he said. I have not spoken to him in year, not had a phone call or an email. We just lost contact. He got on with his life and I moved on. Mr Calderon said he doesn't know if he will go to his step son's funeral. That is going to be so hard but it will be up to his family in New York. I don't know if I will be invited or if I attend. Victim: Arthur Daniels, right, was a sub-contractor relocating and installing office furniture at government offices