Flags fly at half-staff in Washington - and around the U.S. - a day after a tragic mass shooting at the U.S. Navy Yard that left 13 people - including the suspected shooter - dead.

On Tuesday, security was tight - at the base which was closed to everyone but essential personnel. Military police were stationed at the four entrances - checking the vehicles and identifications of the employees allowed back in.

Officials have named 34-year-old Aaron Alexis - a naval contractor from Fort Worth, Texas -- as the sole suspect in the massacre.

Valerie Parlave - the assistant director in charge of the FBI's Washington field office - said Alexis gained entry to the Naval Yard with a valid id.

SOUNDBITE: Valerie Parlave, assistant director in charge of the FBI's Washington field office, saying:

"At this time, we believe that Mr. Alexis entered Building 197 at the Navy Yard with a shotgun. We do not have any information at this time that he had an AR-15 in his possession. We also believe Mr. Alexis may have gained access to a handgun once inside the facility and after he began shooting."

A federal law enforcement source told Reuters Alexis had a history of mental problems but gave no details ... as investigators search for a motive.

Meanwhile, the mourning begins....as U.S. Defense Secretary Chuck Hagel lays a wreath at the U.S. Navy Memorial in honor of the victims of the shooting.