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Monday’s onslaught at a single building 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.

It put all of Washington on edge. Mayor Vincent Gray said there was no indication it was a terrorist attack, but he added that the possibility had not been ruled out.

“This is a horrific tragedy,” he said.

Alexis carried three weapons: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene, according to two federal law enforcement officials who spoke on condition of anonymity because they were not authorized to discuss the investigation.

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 shooting was the work of a lone gunman, and the 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.

[np_storybar title=”Partial list of victims” link=””]

Police in Washington have released the names of seven of the 12 victims killed in Monday’s shooting rampage at Washington Navy Yard

Michael Arnold, 59

Sylvia Frasier, 53

Kathy Gaarde, 62

John Roger Johnson, 73

Frank Kohler, 50

Kenneth Bernard Proctor, 46