Before Terry Durham could even see the gunman's face, she could see the barrel of his rifle aimed straight at her.
"He was far enough down the hall, that we couldn't see his face," Durham told ABC News affiliate WJLA-TV. "But we could see him with the rifle, and he raised and aimed at us and he fired."
Durham and her colleagues at the Washington Naval Yard were just settling into their desks on an otherwise sleepy Monday morning when they heard three distinctive "pops," gunshots, growing increasingly louder and closer.
They ran into a darkened hallway on the third floor of Building 197, turned a corner and were confronted with a man in fatigues carrying a long gun.
The shooter fired on them but missed, hitting "high on the wall" above their heads.
At least 12 people, including a gunman, died in the morning carnage.
Witnesses describe a scene of chaos cascading through the floors of the building, as co-workers hearing sounds of gunfire helped each other to take shelter or flee the building.
"We heard what sounded like muffled gun shots inside, and then we had a person telling us to get out of building," said Tim Jirus, a U.S. Navy commander.
As Jirus was evacuating, he heard two gunshots.
"I looked at the direction where the gun shots were coming from. I looked down, and the guy next to me (who) was standing talking to me, was down in front of me on the ground," he told WJLA.
"I feel very lucky to be alive… [the] guy next to me got shot and I didn't."
Witnesses describe small acts of quick-thinking, selflessness and team work that saved lives.
"Everyone said, 'This is no drill, go, go, go,'" said Frank Putzo, an attorney, describing the atmosphere after hearing several shots. "And a whole bunch of us were able to make it to the emergency exits."
When gun shots rang out in the building's commissary, food-service workers offered to shelter customers out of sight in the kitchen.
On the third floor, employees gathering for a staff meeting chose to barricade themselves rather than flee into the hallway where they heard gunshots.
Capt. Mark Vandroff and his colleagues stacked chairs and desks in front of a door and got down on the floor. Two bullets care through the wall off the conference room, but high above their heads. The stayed in place for 30 minutes before police evacuated them.
"We were hunkered down, we were on the floor because we had heard the previous gunshot. We heard gunfire and we looked up and there were two bullet holes in the top of the wall of the conference room," Vandroff said.
Rather than make noise by talking, his team texted each other from their smart phones.
Outside the building, beyond the police barricades co-workers comforted each other and were met by relieved loved ones. One man found his daughter, who had been shot in the head, already being treated by paramedics. Doctors later described her being "very, very lucky."
But among the crowd were others who had not heard from their family members.
"I'm hoping and praying he [comes] back into my life like he left this morning," said Jacqueline Alston, whose partner Ernest Johnston worked as a janitor at the Navy Yard.
"I'm worried sick, I'm very, very worried," she told ABC News early that day. "I'm numb, I'm numb."
Later Monday evening, while the building remained in lockdown, Alston finally heard from Johnston.
"At 12:00 or 1:00 p.m., I had tears in my eyes. Now i'm so happy i can't even shed a tear," she said. "As soon as I heard his voice I was OK... This has been a trying day today and I'm just praying for those who have lost family."
ABC News' Lisa Stark, Shushannah Walshe and John Parkinson contributed to this report