Rick Mason, a program management analyst who is a civilian with the U.S. Navy, said a gunman was shooting from a fourth floor overlook in the hallway outside his office. He said the gunman was aiming down at people in the building's cafeteria. Mason said he could hear the shots but could not see a gunman.
Shortly after the gunfire, Mason said someone on an overhead speaker told workers to seek shelter and later to head for the gates at the complex.
Patricia Ward, a logistics management specialist, said she was in the cafeteria.
"It was three gunshots straight in a row — pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop, so it was like about a total of seven gunshots, and we just started running," Ward told reporters several blocks away from the Navy Yard.
Ward said security officers started directing people out of the building with guns drawn.
Police and federal agents from multiple law enforcement agencies responded. Ambulances were parked outside, streets in the area were closed and departures from Reagan National Airport were temporarily halted for security reasons.
Janis Orlowski, chief operating officer of Washington Hospital Center, told reporters the hospital was treating three gunshot victims in critical condition. One was Washington, D.C., metropolitan police officer and two were civilian women.
A U.S. Park Police helicopter hovered over the building and appeared to drop a basket with a person onto the roof.
District of Columbia schools officials said 12 public and charter schools and one administrative building in the vicinity of the Navy Yard were placed on lockdown. The action was taken out an abundance of caution, schools spokeswoman Melissa Salmanowitz said.
Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget. Only security personnel were allowed to be armed on the campus.
The Navy Yard has three gates, according to its website. One is open 24 hours a day, seven days a week, and must be used by any visitors. A second gate is only for military and civilian Defense Department employees and the other is for bus traffic.
The Navy Yard is part of a fast-growing neighborhood on the banks of the Anacostia River in southeast Washington, just blocks from Nationals Park.