The Texas man suspected of killing 12 people in a shooting rampage at the Washington Navy Yard received an honourable discharge from the Navy Reserve in 2011.

A US Navy official said Aaron Alexis, a 34-year-old navy contractor, received the honourable discharge even though he exhibited a pattern of misconduct during his career.

Initial reports indicated that Alexis had received a general discharge from the reserve, a category that suggests misconduct.

But the official said he had applied for and received an honourable discharge under the early enlisted transition programme.

A military official said before discharging him honourably, the military had been pursuing a general discharge against Alexis on a series of eight to ten misconduct charges, ranging from traffic offences to disorderly conduct.

Alexis, who had legitimate access and a valid pass to enter the base, was shot dead in a shootout with police.

He had been working as a defence IT subcontractor for computer giant Hewlett Packard, which had been upgrading equipment at the compound.

No motive has been established for the attack, in which about a dozen other people were wounded.

Authorities said none of the dead was an active-duty member of the military and that all were civilians or contractors.

Eight people were injured in the incident, including three who were shot, and all are expected to recover.

Washington authorities have questioned how a US military veteran with a record of brushes with the law could get clearance to enter the base.

"It really is hard to believe that someone with a record as chequered as this man could conceivably get, you know, clearance to get ... credentials to be able to get on the base," said Washington Mayor Vincent Gray.

"Obviously, 12 people have paid the ultimate price for whatever was done to have this man on base."

It has been reported that Alexis had contacted two veterans' administration hospitals recently and was believed to be seeking help for psychological problems.

Military personnel are generally banned from carrying weapons on military installations in the US, but most people with proper credentials are not routinely checked for firearms.