Defense Secretary Chuck Hagel is ordering a security review of all U.S. military facilities around the world after a gunman shot 12 people to death Monday at the Washington Navy Yard.
The Pentagon says details of the review will be announced Wednesday.
The Navy Yard gunman, 34 year-old Aaron Alexis, was fatally shot by police about 30 minutes after he began the shooting rampage. The FBI says he acted alone and got into the Navy Yard with valid identification. His motive is still unknown but police say there is no evidence of connection to terrorism.
Investigators say Alexis had paranoia, a sleep disorder, and claimed he heard voices. His family said he had sought treatment for his mental problems.
Recent US Mass Shootings
September 2013: Gunman kills 12 people at a naval facility in Washington before dying in a gun battle with police
December 2012: Gunman kills 26 adults and children at Sandy Hook Elementary School in Connecticut before killing himself
July 2012: Gunman kills 12 people during showing of Batman movie in Colorado
January 2011: Gunman kills six people and wounds U.S. Representative Gabrielle Giffords in Arizona
November 2009: U.S. Army psychiatrist kills 13 people at Fort Hood, Texas
April 2007: Virginia Tech student kills 32 people before committing suicide
April 1999: Two Columbine High School students kill 12 students, one teacher and themselves in Colorado
Alexis' employer, a contractor called The Experts, said he worked on at least six military installations in July and August without incident.
U.S. senators remembered the victims on Tuesday with a prayer and a moment of silence.
At the Navy Memorial near the Capitol, Hagel, Joint Chiefs Chairman Martin Dempsey, and Washington Mayor Vincent Gray placed a wreath honoring the men and women who were gunned down.
President Barack Obama has ordered all flags across the country to fly at half-staff through sunset Friday to honor the victims.
Alexis opened fire Monday on employees of the Naval Sea Systems Command, using a shotgun and a handgun. Authorities say earlier reports he had an assault-style rifle were incorrect.
Alexis was a resident of Fort Worth, Texas. U.S. media reports say Alexis had been arrested in two previous shooting incidents, in 2004 in Seattle, and in Fort Worth in 2010.