It has happened again. Another mass shooting in america. This time just south of the white house and the capitol. At our country's oldest naval facility, the washington navy yard where today's rampage... See More
It has happened again. Another mass shooting in america. This time just south of the white house and the capitol. At our country's oldest naval facility, the washington navy yard where today's rampage claimed more than a dozen lives including this man, aaron alexis, the shooter identified by police late today. Why he opened fire this morning is still a mystery, the lives lost to tragedy, how so many survived, a blessing. Our team is out with those stories tonight. We start with jim avila on the moment of crises. Reporter: A shooting free for all that began even before the suspect wearing black with a tall military bearing marched his way into the navy yard. This man hit while on the street, desperate on lookers trying to revive him as the ambulance arrived. Multiple shots fired. Multiple people down. Everyone said this is no drill. Go, go, go, emergency exhibits now. Reporter: D.C. Metro police say they get their first call of multiple victims down at 8:20 in the morning. By then a security guard in building 197 of this come plek is hit. Shooting down from the walkways over the open core of the five story building into the food court below. The shots were randomly aimed and fortunately not always on target. The first three shots were done pow, pow, pow. Three second later, pow, pow, pow, four more shots. Reporter: By 9:33 the first ambulances rushed victims to hospitals. Officer down on the third floor. Reporter: This ambulance with a police escort carried a d.C. Patrolman who shouted out with a gunman. At 9:45 the faa stops all flights in and out of nearby reagan national airport. BACK IN THE BUILDING EMTs Struggled to safely remove the injured. At least five people shot. They are attempting to bring them outside. Reporter: Only later would police realize the death toll was much higher, a dozen innocents in all, one of the victims standing shoulder to shoulder with navy commander tim gyrus. I saw him drop. He was shot in the head. Reporter: Finally at 12:14 police announce e they've fatally shot the gunman, a former navy man who may have worked at the yard recently as a civilian contractor. Police cordoned off a wide area around the scene for hours. The capitol building was shut down for a while and the nationals called off their night league baseball game as rumors swirled of a multiple gunman. The big concern for us now is that we potentially have two other shooters that we have not located at this point. Reporter: The navy yard staying on lockdown for more than six hours. Survivors told to hide in place. Some barricading themselves in top secret, secure closets, others under their desks. We put chairs and then tables up against the door. Reporter: Navy captain mark van drof was evacuated from his office. We heard gunfire and locked up and there were two bullet holes in the top of the wall of the conference room. Reporter: Tonight police are beginning to say that they are beginning to understand the how of what happened here but not the why. No clues as to the motive and no evidence that this was terrorism. George? Thafrks, jim. Let's get more from two eye witnesses on the scene. Thank you both for joining us in what I know has been a hair owing day. Terry, you're on the third floor, building 197 trying to help some colleagues get through the exit. What do you see next? When we heard the alarm we didn't all know what was happening, so we were just trying to get people to move into the stairwell and get them out of the building. When we saw one of the gunman, at least the one that was on the third floor was down the hall way. He appeared to be a tall man, looked like he might have been in a dark blue navy uniform. He had a rifle, some kind of weapon with him. As we were standing there trying to get our colleagues through the passage way, he pointed the gun at us and shot. Todd, he shot right at you? Yes, he did. Yes, he did. It was as we were going out the door we heard the shots being fired and we realized that the gunman was really close and two or three seconds later he did come around the corner and pointed the gun right at us and fired two or three shots. Terry, was he aiming directly at individual people or just firing randomly? When we saw him he raised the gun and aimed right at us, at a group of us that were standing there trying to wait until we could get through the doorway. And was he saying anything? He did not say a word. Basically we heard the shots and he came around the corner and immediately turned to us and started shooting. How quickly was he firing? Very quickly. He was firing off rounds at least every three or four seconds. Let me ask you both, what do you take away from what has got to be one of the most difficult days of your life? Thankful to be alive. He was far enough away that when he shot at us he did not hit us. And many others were not as lucky. You really have to appreciate life and take every day as it comes. You never think it's going to happen to you. You see it all the time on the news and all of a sudden you go to work on a monday morning and it happens. We're thankful you're okay today and thank you for your time. Thank now to the investigation. Pierre thomas has been tracking the fast moving developments. We are learning more about the gunman. Reporter: An investigation is under way to determine exactly why this suspect snapped. Tonight the fbi is urgently seeking the public's help to find out more about the man they believe is responsible for a massacre, identified as aaron alexis. No piece of information is too small. We are looking to learn everything we can about his recent movements, contacts and association. We ask anyone with information about him to contact us at 1800 call fbi. Military records show alexis was in the navy from may 2007 to january of 2011 serving at the naval air station in fort worth texas. He was arrested by forthworth police for discharging a gun in public. It's unclear whether it played a role in his leaving the navy. Police are investigating whether he recently worked as a civilian contractor at the washington navy yard. Alexis fit the description of eye witnesses who described a cold blooded killer randomly gunning down whoever was in front of him until he was shot and killed by police. The suspect's friends were shocked, among them the owner of a thai restaurant in texas. He's not aggressive. He had a gun but it doesn't mean he's going to shoot people. Reporter: Authorities continue to investigate whether a second suspect may have been involved but sources tell abc news the hard evidence points to only one shooter. As of tonight officials found no evidence indicating terrorism. The prime theory is that he had a grudge against the navy. The fbi is asking the public to help them find out a lot more about this gunman? Until they can dissect every aspect of the suspect's life nothing is ruled out. That's why they are so desperate for the public's help. They're tracking his movements, did he go overseas, who was he in contact with in the final days. Martha raddatz covers all things military for us. You've been to this facility many times. It's a fairly secure facility. But not as high level as secures basis where there are ships and war fighting equipment. At the navy yard employees have badges to gain entrance but the vehicles are not searched. If you are a visitor you would have to have an escort of some kind and your name on a list. It's an office complex basically but a lot of senior officials work there. Even though this is a huge office complex the exception is that the home of the chief of naval operations is there, admiral jonathan greenard. He's one of the joint chiefs of staff. He and his wife were evacuated right after the shooting. The majority of the other chiefs live at joint base meyer in arlington virginia and that base does have checks. We're not sure how the suspect got into the navy yard.
This transcript has been automatically generated and may not be 100% accurate.