"They are under lockdown because they just don't know," Reyes said. "They have to check every building in there, and they have to check every room and just, of course, a lot of rooms and a lot of buildings."
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 grounds, but that can include uniformed security officers, civilian contractors and members of the Naval Criminal Investigative Service.
Cmdr. Timothy Jirus said he has a clearance that would allow him to carry a gun on the campus. He has a secure access card that he swipes to get into the headquarters office.
"I think the security is really good, up until today," Jirus said.
Everyone must show an ID to get through a main gate, and at the building entrance, everyone must swipe a badge to pass through either a door or gate, depending on the entrance.
That "makes me think it might have been someone who works here," Mason said.
The Navy Yard has three gates, according to its website. One is open around the clock and must be used by visitors. A second gate is only for military and civilian Defense Department employees. The third gate is for bus traffic.
The Navy Yard is part of a fast-growing neighborhood on the banks of the Anacostia River in southeast Washington, blocks from the Nationals Park baseball stadium.