"We don't have any evidence or indication at this stage that there was another shooter, even though we haven't completely ruled that out," said Washington mayor Vincent Gray.
Police chief Cathy Lanier stressed that: "we do now feel comfortable that we have the single and sole person responsible for the loss of life inside of the base today.
"If anything changes as we continue to go forward, we certainly will make that available to the public," she told a briefing, "but, ... again, we are lifting the shelter in place" warning.
Police have identified the alleged lone gunman as Aaron Alexis of Forth Worth, Texas, who served in the Navy from 2007 to 2011 before becoming a defense subcontractor for computer giant Hewlett-Packard.
He was killed in an exchange of gunfire with police.