The United States Navy Secretary has ordered a review of security at all Navy and Marine Corps installations in the wake of the shooting rampage by a former Navy reservist at a Washington DC naval base.
Aaron Alexis, 34, killed 12 people and wounded eight before police shot him dead at the Washington Navy Yard.
Reports say Alexis had received treatment for mental health issues.
He had been given clearance to enter the base as a technical contractor, despite two gun-related brushes with the law and a discharge from the Navy Reserve in 2011 after a series of misconduct issues.
The Associated Press says he had been treated for paranoia, trouble sleeping and hearing voices.
On Tuesday, the US capital remained in a state of shock and mourning.
The BBC reports that Defence Secretary Chuck Hagel laid a wreath at US Navy Memorial Plaza in honour of the Navy Yard victims.
A Washington Nationals baseball game originally scheduled for Monday night was rescheduled to Tuesday afternoon - the baseball stadium is just blocks from the Navy Yard.
The flag flew at half-mast over the White House, where hours after the attack President Barack Obama mourned "yet another mass shooting".