Authorities are questioning how a U.S. military contractor with a history of violence and mental problems could have gotten clearance to enter a Navy base in Washington where he killed 12 people Monday before police shot him dead.
Police say they have found no motive for 34-year-old Aaron Alexis' shooting rampage at the Washington Navy Yard. They also say they have found no evidence of a connection to international terrorism, or any political or religious motivation for his actions.
Alexis had clearance to enter the base despite two gun-related brushes with the law, and he had been discharged from the U.S. Navy Reserve two years earlier after a series of misconduct issues.
News reports say investigators found Alexis had paranoia, a sleep disorder and was hearing voices. He had been treated since August by the Veterans Administration for his mental problems.
Early Tuesday, U.S. senators remembered the victims with a prayer and a moment of silence. Later, at the Navy Memorial near the U.S. Capitol, Defense Secretary Chuck Hagel, Joint Chiefs Chairman Martin Dempsey, Washington Mayor Vincent Gray and other senior officials placed a wreath honoring the men and women who were gunned down.
Alexis opened fire Monday on employees of the Naval Sea Systems Command, using a shotgun and a handgun he is believed to have taken from a wounded security officer.
In addition to the dead, eight people were hurt in Monday's incident. All are expected to survive.
Police released the identities of seven of those killed late Monday, with their ages ranging from the late 40s to early 70s.
Alexis is a New York City native and a resident of Fort Worth, Texas. News outlets say Alexis had been arrested in two previous shooting incidents, in 2004 in Seattle and in Fort Worth in 2010.
Late Monday, grieving residents gathered outside the Naval Yard and held a silent candlelight vigil.
President Barack Obama has ordered all flags across the country to fly at half-staff through sunset Friday to honor the victims.