Washington (CNN) -- When a fire alarm blared Monday morning at the Washington Navy Yard, Terrie Durham thought it might be another drill.
Then office fire wardens came in shouting for everyone to get out as fast as possible, and "that's when we started moving," she told CNN affiliate WJLA.
Durham and colleagues including Todd Brundidge rushed out the door toward the exit. Down the hall, they saw a man with a rifle.
"He stepped around the corner and we heard shots," Brundidge said. What seemed like two or three seconds passed, and then "he aimed his gun at us and then he fired at least two or three shots."
The pair provided an eyewitness account of a suspect they described as a tall black man accused by authorities of opening fire at the heavily secured Navy yard along the Anacostia River, not far from the U.S. Capitol and just blocks from where baseball's Washington Nationals play.
Durham said the gunman was too far away to see his face, but "we could see him with the rifle and he raised and aimed at us and fired." The shots hit high on the wall, and the group ran out of the building.
Navy Cmdr. Tim Jirus didn't see a gunman, but he witnessed the carnage.
He told CNN he was in an alley outside Building 197 to help people get down the fire escapes. A worker from the building approached him to say he heard someone was shooting inside.
Then he heard more gunfire -- two shots that apparently came from the building. The man who stood three feet from him fell to the ground, bleeding from a head wound, "and that's when I ran," Jirus said.
"I never met him before in my life," said Jirus, who described himself as "lucky to be here."
He had not heard clear gunshots earlier, but this time, he was certain.
"In the alleyway with the amount of sound reverberating, you know exactly what it is, not to mention he's on the ground, bleeding," Jirus said.
The shootings that began around 8:20 a.m. set off chaotic scenes inside the Navy Yard and in the redeveloped neighborhood around it, with the FBI and other law enforcement agencies converging in what seemed like a tidal wave of sirens and helicopters.
Police Chief Cathy Lanier praised the speed and cooperation by various law enforcement agencies that responded.
After the first call, D.C. police were on the scene "within literally two to three minutes," she told reporters.
"Now, internal security had already engaged, identified and engaged the suspect," Lanier continued. "We already had victims down at that point. Within seven minutes, we had active shooter teams inside the building, moving through the building."
The first responders found "one of the worst things we've seen" in Washington," according to Lanier.
"As officers entered the building, they were, you know, making transmissions in and keeping command informed as to what they were coming across as they went through," she said. "Multiple victims. There was gunfire still going on."
In the bureaucratic tone of police talk, she described a sporadic gun battle -- "mulitple engagements" -- that witnesses said lasted 40 minutes or so.
"Both metropolitan police and park police, on preliminary information, got into a final gun battle with the suspect," Lanier said, adding there were "multiple incidents in that time frame in which there was a gunfight with a suspect," who now was "deceased."
Meanwhile, workers poured out of the huge Navy complex into the surrounding streets.
"Everybody was going down the stairs," Brundidge said in describing the pandemonium. "People were pushing. People were shoving. People were falling down. After we came outside, people were climbing the wall, trying to get out over the wall. ... It was just crazy."
Paul Williams, a program coordinator at a nonprofit that works with foster children, was heading to his office nearby when he encountered what he described as a panic.
"I had my headphones in, so I didn't know what was going on," Williams, 29, told CNN. "I was listening to music, but I heard four rapid bangs—bang, bang, bang, bang."
At first he thought it was construction noise that has become common in the area, but about 30 seconds later, he saw a rush of people coming toward him from the Navy Yard.
"I didn't know what was happening. I just ran with them," Williams said, adding that "everyone seemed scared. People were crying, people were being consoled and calling loved ones and family."
The impact of the shootings spread through Washington. The Department of Transportation was locked down, as were some U.S. Senate offices, and the Federal Aviation Administration temporarily halted air traffic Monday morning at Reagan National Airport.
Two events Monday night -- a Nationals baseball game against the Atlanta Braves and a Musica Latina event at the White House -- were postponed.
To Brundidge, the encounter with a gunman was hard to believe.
"You know," he said, "you just go to work and you never think something like this is going to happen in your building."
CNN's Rene Marsh, Ashley Killough, Gabe Lamonica and Virginia Nicolaidis contributed to this report.