A U.S. Park Police helicopter patrols over the Navy Yard on Sept. 16. (Paul J. Richards/AFP/Getty Images)

Behind its white brick wall and the iconic, centuries-old Latrobe Gate, the historic Washington Navy Yard, scene of Monday’s deadly shooting, has undergone a major renovation and expansion over the past 15 years.

Somewhat moribund by the 1990s, its grounds contaminated by more than a century of service as a major Navy gun factory, the Navy Yard had enjoyed a renaissance in recent years as the bustling home to several major commands and offices, including the Naval Sea Systems Command, where the shooting was centered.

The Navy Yard, one of the oldest military installations in the nation, has about 16,000 employees, including civilian and military, who work in a mixture of old brick buildings and more modern structures spread across the grounds along the Anacostia River. Until Monday, many considered it safe.

“After 9/11, being behind the wall seemed like a good place to be,” said retired Navy Capt. Charles T. Creekman, who has spent 20 years at the Navy Yard as an officer and now as executive director of the Naval Historical Foundation.

The workforce includes Navy and Marine Corps uniformed personnel but is predominately civilian. The sidewalks and the food court generally bustle with workers, among them Navy contractors, engineers, lawyers, historians and procurement officials, as well as tourists visiting the National Museum of the U.S. Navy.

Sea Systems, which employs about 3,000 workers at the Navy Yard, is the largest of the Navy’s five systems commands. With responsibility for engineering, building and procuring ships, submarines and weapons, Sea Systems’ annual budget approaches $30 billion.

The command’s headquarters in Building 197, where the shooting erupted, is a historic brick facility used during World War II to assemble the 16-inch guns used by battleships. It was renovated and officially made headquarters in 2001 as part of a $200 million construction project involving several buildings that were part of the U.S. Naval Gun Factory.

The Navy Yard is home to the Navy’s top officer, Adm. Jonathan Greenert, the chief of naval operations, who resides in the historic Tingey House adjoining Leutze Park, the ceremonial square lined with bronze cannons from the Barbary Wars and other conflicts.

After the shooting at 8:20 a.m., an emergency public address system known as Big Voice and a flood of e-mails and text messages directed employees to stay put. The first message, at 8:28 a.m, was to shelter in place.

“Stay indoors, if outside, get within a building,” read an e-mail sent at 9:15 a.m. “Stay away from windows or open buildings, Remain indoors until the ‘all clear’ is given. Secure doors and windows.”

Greenert and his wife, Darleen, were safely evacuated from the complex.

“Our team of sailors and Navy civilians at the Navy Yard deserve our care and concern at this time,” Greenert said, according to a Navy statement.

Hundreds of workers, including Creekman, were eventually assembled in the food court, where they awaited further instruction.

The three vehicle gates at the Navy Yard are guarded by U.S. Marines and personnel from the Naval District Washington Security, including some civilians. Security procedures Monday morning seemed normal, according to employees.

Visitors without military identification must have a valid reason for entry to the Navy Yard and show identification to gain access, according to the Navy. Entry to the Sea Systems headquarters requires a badge.

The Washington Navy Yard, established in 1799, served briefly as home port for the new nation’s fleet and was a bustling shipyard in the capital’s early years. When Washington was captured by British forces in August 1814, the Navy Yard was burned at the order of U.S. commanders who did not want ships and supplies to fall into enemy hands.

The Navy Yard was rebuilt with a taller wall, to keep out looters who took advantage of the chaos to pick over the premises.

The rebuilt yard took on a new role as a Navy center for ordnance and technology, including torpedoes and steam engines, and during the Civil War, was a key part of Washington’s defenses. During World War II, the yard was the largest naval ordnance plant in the world, employing 25,000 people in 188 buildings.

The munitions and industrial work was phased out and the gun factory closed in 1962. In subsequent years, the Navy Yard served mainly as office space for clerical work.

In 1983, a Puerto Rican separatist group set off a bomb in a computer center, but the blast caused no injuries. Adm. Mike Boorda, then chief of naval operations, committed suicide at the Navy Yard in 1996.

But in the 1990s Congress agreed to move several major commands to the Navy Yard, which brought a spate of construction, new buildings and activities, environmental cleanup and thousands of workers.

In addition to Sea Systems, the Washington Navy Yard is home to the Naval Facilities Engineering Command, the Military Sealift Command, the Office of the Naval Inspector General and the Office of the Judge Advocate General.

Late Monday, many workers were still gathered at shelter points, awaiting police escort from the complex.