36 Photos Shootings at the Washington Navy Yard
VIDEO: Alleged Navy Yard shooter targeted victims from high angle
Video Why did Navy Yard gunman open fire?
The motive for the attack — the deadliest mass shooting on a military installation in the U.S. since the tragedy at Fort Hood, Texas, in 2009 — was a mystery, investigators said.
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Mayor Vincent Gray said there was no indication it was a terrorist attack, but he added that the possibility had not been ruled out.
The assault at the highly secure Navy Yard unfolded about 8:20 a.m. in the heart of the nation's capital, less than four miles from the White House and two miles from the Capitol.
The gunman, Aaron Alexis, a 34-year-old former Navy reservist whose last known address was in Fort Worth, Texas, died after a running gunbattle inside the building with police, investigators said.
A law enforcement official told CBS News' Pat Milton the shooter was discovered with an AR-15 assault rifle and a shotgun with him and was wearing dark blue clothing.
Milton reports that the shotgun was traced to a gun store in Lorton, Va., according to a federal law enforcement official. The official said the shotgun was purchased at the store within the last week.
CBS News correspondent Bob Orr reports that Alexis was also found with a sidearm that authorities believe may have come from one of the Navy Yard's security officers.
For much of the day, authorities said they were looking for a possible second attacker who may have been disguised in an olive-drab military-style uniform.
But by late Monday night, they said they were convinced the attack was the work of a lone gunman, and the security lockdown around the area was eased.
"We do now feel comfortable that we have the single and sole person responsible for the loss of life inside the base today," Washington Police Chief Cathy Lanier said.