The suspected gunman was employed by the defence industry and used his pass to get into the Washington Navy Yard on Monday.
He went on a deadly shooting rampage, spraying bullets in the hallways and firing from a balcony on workers in an atrium below, killing 12 other people.
US officials spent much of the time after the shooting looking for a possible second gunman. But a news conference late on Monday said that there was no evidence another gunman was involved.
“We have continued to pursue the possibility of there being another shooter but we don’t have any evidence, any indication, at this stage that there was another shooter,” said Washington Mayor Vincent Gray.
Aaron Alexis, 34, was an information technology employee and former US Navy reservist whose last known address was in Fort Worth, Texas.
He died after a running gun battle inside the building with police, said investigators said.
Evacuees raise their hands as they are escorted from the scene of a shooting at the Washington Navy Yard in Washington
The gunman carried three weapons: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene, two federal law enforcement officials told Reuters, on condition of anonymity.
Speaking about “yet another mass shooting” in the US, that had killed American “patriots”, President Barack Obama promised to make sure “whoever carried out this cowardly act is held responsible”.
Monday’s attack is the deadliest shooting at a US-based military installation since Major Nidal Hissan, an US Army psychiatrist, killed 13 people and wounded 30 others in 2009 at Fort Hood in Texas. He was convicted last month and sentenced to death.