The suspect in the Washington Navy Yard shooting, Aaron Alexis, used a valid pass to gain entry to the base, the FBI said on Tuesday.
"Mr. Alexis had legitimate access to the Navy Yard as a result of his work as a contractor and he utilized a valid pass to gain entry to the building," said Valerie Parlave, assistant director in charge of the FBI's Washington field office.
Authorities believe Alexis entered the base's Building 197 with a shotgun and may have gained access to a handgun inside.
There is no information that he had an AR-15 semi-automatic rifle, Parlave said, adding that the shotgun was bought legally in Virginia.
Authorities said the suspect killed 12 people on Monday before police shot him dead.
White House to review contractors after Washington shootings
The White House said Tuesday it will review standards for federal government contractors after the Washington Navy Yard shootings, which authorities said were committed by a contract worker who had a security clearance to work there.
President Barack Obama will be briefed on Tuesday on the details of the Washington Navy Yard shootings by his national security aides, including Attorney General Eric Holder and FBI Director James Comey, White House spokesman Jay Carney said.
Monday's attack left 13 people dead, including the suspected gunman, 34-year-old Aaron Alexis, a military reservist with a history of violence and mental problems.
Alexis was a Navy contractor who had been given clearance to enter the base despite two gun-related brushes with the law and a discharge from the Navy Reserve in 2011 for "misconduct issues."
The White House Office of Management and Budget is looking at issues surrounding security clearances for contractors and federal employees, Carney said.
"At the president's direction, OMB is examining standards for contractors and employees across federal agencies," Carney told reporters at a briefing.
"This is a matter that the president believes and has believed merits review," he said, noting the Director of National Intelligence had previously begun a review of security clearance policies for certain types of government contractors.
