Why Did He Do It? Navy Yard Gunman Was Military Contractor, Former Sailor

This story has been updated.

The suspect in the shooting at the Washington Navy Yard that killed 12 people on Monday morning has been identified as Aaron Alexis, a 34-year-old former Navy petty officer from New York City.

Alexis, who lived in Fort Worth, Texas, and worked as a military contractor, was shot and killed in the rampage.

Law enforcement officials said they’re still investigating a motive and the possibility that a second shooter was involved. Washington, D.C. Mayor Vincent Gray said officials “don't have any reason at this stage to suspect terrorism, but certainly it has not been ruled out."

A friend in Fort Worth told CNN that Alexis had been upset with his employer over “a payment issue.”  He was working for a subcontractor on a major Navy information technology project with HP Enterprise Services. In a statement, HP said: “We are deeply saddened by today’s tragic events at the Washington Navy Yard. Our thoughts and sympathies are with all those who have been affected. Aaron Alexis was an employee of a company called ‘The Experts,’ a subcontractor to an HO Enterprise Services contract to refresh equipment used on the Navy Marine Corps Intranet (NMCI) network. HP is cooperating fully with law enforcement as requested.”

Navy officials said he was discharged from Fleet Logistics Support Squadron (VR) 46 in Fort Worth in early 2011 and he was discharged over “a pattern of misconduct.”

In addition to the 13 dead, about a dozen others were wounded, officials said. The entire area around the Navy Yard, located in southwest D.C., has been closed off. Federal buildings across the district, including the Pentagon, are on heightened security. The Senate was on lockdown as SWAT teams and police searched for two possible other shooters. One man was identified and cleared, but Metropolitan Police say they’re still investigating whether Alexis had help from another shooter, who was described by witnesses at the scene as wearing a military-style uniform.

“[Defense Secretary Chuck Hagel] has assured the Navy we will provide any resource or capability needed to get the Washington Navy Yard community through this event,” Pentagon Press Secretary George Little said. "This is a fluid situation. Navy officials are working closely with law enforcement and emergency management representatives from the FBI and the District of Columbia to secure the scene and begin the investigation. "

The Navy Yard is expected to be closed on Tuesday, according to Government Executive.

At 8:20 a.m. EST, an “active shooter” was reported inside the Naval Sea Systems Command Headquarters building, located at Bldg. 197 at the Navy Yard. The Naval Sea Systems Command's headquarters employs about 3,000 people, where the U.S. Navy builds, buys and maintains ships, submarines and combat systems.

Chief of Naval Operations Adm. Jonathan Greenert, who lives at the Navy Yard complex, was evacuated from the building after the shooting began, officials told Defense One.

Todd Brundidge, an executive assistant with Navy Sea Systems Command, told The Associated Press that 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.

Speaking at an event on the economy in Washington, D.C., not far from the Navy Yard, President Obama vowed that "whoever carried out this cowardly act is held responsible.”

"We still don't know all the facts, but we do know several people have been shot and some have been killed," Obama said. "We're confronting yet another mass shooting and today it happened on a military installation in our nation's capital. It's a shooting that targeted our military and civilian personnel."

Kevin Baron contributed to this report.