At least one gunman opened fire inside a building at the Washington Navy Yard on Monday morning, and officials said six people were killed and at least four were wounded, including a law enforcement officer.

Police were looking for two other potential gunmen wearing military-style uniforms, including one who had on a beret, chief Cathy Lanier said. One had a long gun and the other was also armed, she said. One of the three gunmen had died, though Lanier didn’t say how.

Witnesses described a gunman opening fire from the fourth floor, aiming down on people in the first-floor cafeteria. Others said a gunman fired at them in a third-floor hallway. It wasn’t clear whether the witnesses on different floors were describing the same gunman.

As witnesses emerged from the building, a helicopter hovered over it, schools were on lockdown and airplanes at nearby Reagan National Airport were briefly grounded so they wouldn’t interfere with law enforcement helicopters. Less than 2 miles away, security was beefed up at the Capitol, but officials said there was no known threat there. President Barack Obama was getting frequent briefings on the shooting.

Two Navy officials confirmed at least six people had died. The officials spoke on condition of anonymity because they were not authorized to discuss the situation publicly.

About 3,000 people work at the Naval Sea Systems Command headquarters, which builds, buys and maintains the Navy’s ships and submarines and combat systems.

Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other co-workers encountered a gunman in a long hallway of their building on the third floor. The gunman was wearing all blue, he said.

“He just turned and started firing,” Brundidge said.

Terrie Durham, an executive assistant with the same agency, said she also saw the gunman firing toward her and Brundidge.

“He aimed high and missed,” she said. “He said nothing. As soon as I realized he was shooting, we just said, ‘Get out of the building.'”

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.

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(AP)