The suspected gunman who killed 12 people before being shot dead by police in a military complex in Washington had been honourably discharged in 2011, officials claim.
Former Texas resident Aaron Alexis, 34, was identified as the sole gunman involved in the shooting rampage at the Washington Navy Yard.
Mr Alexis served in the US Navy reserves from May 2007 until January 2011, a US official said.
A navy official said he had received an honorable discharge from the navy reserve in 2011, even though he exhibited a pattern of misconduct during his career.
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But the navy official said Mr Alexis had applied for and received an honorable discharge under the early enlisted transition programme.
His motive still remain unknown but US officials said that he had a sleep disorder and was hearing voices in his head.
Police have released the names of seven of the 12 victims killed at the Washington Navy Yard.
* Michael Arnold, 59
* Sylvia Frasier, 53
* Kathy Gaarde, 62
* Roger Johnson, 73
* Frank Kohler, 50
* Kenneth Bernard Proctor, 46
* Vishnu Pandit, 61
The names of the four other victims have been withheld until their next of kin could be notified.
The officials also said there has been no connection to international or domestic terrorism, and investigators have found no manifesto or other writings suggesting a political or religious motivation.
Monday’s attack was the deadliest on a military installation in the US since 2009 and a reminder of the country’s long debate over guns.
Family members told investigators that Mr Alexis had been treated since August by the federal Veterans Administration, the officials said.
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They spoke on condition of anonymity because the criminal investigation was continuing.
Mr Alexis used a valid pass to get into the Washington Navy Yard.
In the past, he had complained about the navy and being a victim of discrimination and had several incidents with law enforcement, including two shootings.
Mr Alexis carried three weapons in the attack: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene, according to two officials who spoke on condition of anonymity because they were not authorized to discuss the investigation.
The AR-15 is the same type of rifle used in last year’s mass shooting at a Connecticut school that killed 20 students and six adults.
The weapon was also used in the shooting at a Colorado movie theatre that killed 12 and wounded 70.
Authorities said they were convinced that Mr Alexis had acted alone.
Eight people were hurt, including three who were shot and wounded, Mayor Vincent Gray said. They were all expected to survive.