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A former US naval reservist killed 12 people and wounded 14 in a shooting rampage at a military base in the heart of Washington on Monday before being killed in a shootout with police.

Police identified the gunman as Aaron Alexis of Fort Worth, Texas, who served in the Navy from 2007 to 2011 before becoming a defense subcontractor for computer giant Hewlett-Packard.

US authorities investigating the shooting spree at the Navy Yard said the gunman appeared to have acted alone.

"We do now feel comfortable that we have the single and sole person responsible for the loss of life inside of the base today," police chief Cathy Lanier said late Monday.

The Federal Bureau of Investigation appealed to the public for information on the 34-year-old,whose military service was marked by disciplinary problems and who reportedly had once been arrested but not charged in Texas for shooting a bullet through his apartment ceiling.

The FBI released a photo of Alexis, who held the rank of an aviation electrician's mate 3rd class and had served full time in a logistics support squadron in Fort Worth, according to the Navy.

Alexis carried three weapons: an AR-15 assault rifle, a shotgun and a handgun that he took from a police officer at the scene, according to two federal law enforcement officials who spoke on condition of anonymity because they were not authorized to discuss the investigation.

Monday's shooting left Washington on edge and there was a security scare hours later at the White House when a man who apparently threw fire crackers over a fence at the US president's residence was swiftly arrested.

Even hours after the mass shooting - the deadliest on a military installation in the US since the tragedy at Fort Hood, Texas, in 2009 - Alexis' motivation for opening fire was unclear. His four-year stint in the Navy was troubled, officers said.

"There is definitely a pattern of misconduct during his service," a US military officer said on condition of anonymity.

Friends in Texas told US media that Alexis had an interest in Buddhism and was conversant in the Thai language and had thought about moving to Asia.

Most recently, Alexis was employed as an IT subcontractor for a company called "The Experts", which was working on a Hewlett-Packard contract to upgrade equipment for an intranet network used by the US Marine Corps and Navy, HP said in a statement.

His work as a naval contractor suggested he may have had a pass that could gain him entry to the Naval Sea Systems Command, which oversees shipbuilding programs carried out by defense firms.

It was unclear if the military or HP had been aware of Alexis' brushes with the law, including reportedly two shooting incidents, before he was hired for the IT job.

The shooting sparked a massive show of force as police and federal agents descended on the Navy Yard on the Anacostia River, less than 3 kilometers from the Capitol.

US President Barack Obama ordered that flags be flown at half-staff in the US capital until Friday as a mark of respect for the dead.

（中國日報網英語點津 Helen 編輯）

About the broadcaster:

Lance Crayon is a videographer and editor with China Daily. Since living in Beijing he has worked for China Radio International (CRI) and Global Times. Before moving to China he worked in the film industry in Los Angeles as a talent agent and producer. He has a B.A. in English from the University of Texas at Arlington.