An Emergency Response Team vehicle arrives to the scene where a gunman was reported at the Washington Navy Yard in Washington, on Monday, Sept. 16, 2013.
Witnesses to Monday's Navy Yard shooting that left 12 victims and the shooter dead describe a scene of disbelief, and then chaos, when the gunman opened fire in Building 197.
It happened without warning, said Patricia Ward, a Navy logistics specialist who was on the first floor getting breakfast when she heard shots on the fourth floor.
"It was like pow pow pow… and then a few seconds it stopped and then pow pow pow… so we just ran," she recalled. "A lot of people was just panicking, there were no screams or anything because we were in shock."
Rick Mason was on the fourth floor, where the gunman allegedly opened fire. He says he initially thought "it" was workers using equipment.
"Noises were repeating themselves like four or five shots, one of the managers came out and said 'Leave the building, leave the building because there is shooting happening," he said.
Bryan Chaney was on the second floor: "I heard the first shot, I didn't it was." He didn't think anything of it. But then he saw the security guards, he noticed some sort of commotion.
"I walked through the double doors… I heard pop pop pop … in succession … those guys were like … that was gun shots," he said.
It happened without warning. A mass shooting in a D.C. military building. More than a dozen killed, all of them civilians. Witnesses, family members, authorities all asking the same question: why did it happen?