Navy officer Tim Jirus told reporters the man from the Washington Navy Yard's maintenance department was felled by gunshots as workers scrambled out of buildings following reports of a shooter on the rampage.
Jirus told CNN he had evacuated workers in his department after hearing what sounded like "muffled shots" from a different part of the base.
"It sounded like a cap gun going off. A small caliber, if anything. Then about a minute or two after that, somebody was running through the hallway saying 'Hey, everybody, get out of the building,'" Jirus said. "I wanted to get everybody out of the building. We walked out."
While struggling to understand what was going on, a man from the base's maintenance department spoke to Jirus.
"He walked up and told me he heard there was a shooter in our building. We were just standing here maybe three feet away having a conversation and we heard two more gunshots and he went down. That's when I ran," Jirus said.
"I'm fairly certain he was dead because he was shot in the head," he added. "It's traumatic. I don't feel lucky he got hit instead of me but I feel lucky to be here."
Another witness told NBC News how she saw the gunman silently open fire on her and co-workers -- but miss.
"We're lucky he was far enough away he was a bad shot," Terrie Durham said.
Durham, a civilian employee, said she was evacuating her third floor office when she saw the gunman standing about 40 yards away at the opposite end of a hallway.
"He was a tall man, appeared to have dark skin, looked like he was in some kind of uniform and he had a rifle," Durham told NBC.
"And he aimed at us and shot but missed, thank God."
One of Durham's co-workers, Todd Brundivge, said the gunman had acted with a chilling detachment.
"No words. He raised the gun and started firing," he told NBC. "He said nothing."
US authorities said Monday another gunman may have been involved in the attack. One shooter was believed to be dead at the scene while two more male suspects wearing military-style uniforms may still be at large, police said. One of those men was later eliminated as a suspect.
Jirus meanwhile said personnel at the base are required to have key cards to access the building but visitors were required to check in with security.
"It will be interesting to see as this develops, who the shooter is, how he got in, those type of questions answered," he said.
Another witness, Patricia Ward, said she had just paid for breakfast at the base's cafeteria when the carnage erupted.
"I was waiting for my friend to pay when we heard the gun shot. It was three gun shots straight in a row -- 'pow, pow, pow'. Three seconds later i was 'pow, pow, pow, pow' so like a total of seven gunshots," Ward said. "We just started running."
Ward said workers at the base were not required to pass through metal detectors to get into the facility.
"You have to scan your badge, there is no metal detector," she said.
The shootings sent the US capital onto a state of maximum alert, with the immediate vicinity surrounding the Navy Yard locked down for several blocks in all directions.
Residents of the neighborhood were told to stay inside while authorities set up checkpoints at various intersections as the sky buzzed with with helicopters overhead.
Several schools were placed on lockdown while at the US Senate business was adjourned out of an "abundance of caution."
