WASHINGTON -- The civilian contractor suspected of killing 12 people in Monday's assault at the Navy Yard sought assistance for mental illness as recently as a month ago, a federal law enforcement official said Tuesday.

Aaron Alexis, who was killed by police responding to the massacre, reported symptoms of paranoia including hearing voices, said the official who is not authorized to comment publicly.

It was not believed that he was ever declared mentally ill by a court. Such a finding would have prohibited him from purchasing a weapon.

The official said the 34-year-old suspected shooter paid about $540 to buy a 12-gauge shotgun and ammunition in recent days at a gun store in Virginia and took them to the Navy Yard Monday shortly before authorities believe he carried out the assault.

The official said that investigators are just beginning to analyze Alexis' possessions to determine if it might reveal any motive for the slayings.

In a newly revised sequence of events, federal investigators believe Alexis cleared a security checkpoint with his contractor ID and carried the unassembled weapon into Building 197 at the Naval Sea Systems Command headquarters.. The official said that investigators now believe that he stopped in a men's room where he assembled the weapon and proceeded to a site on the third or fourth floor of the building that overlooked an interior atrium.

It is believed, the official said, that Alexis began firing indiscriminately on the people below with the law enforcement-style shotgun. After firing several rounds, the official said, Alexis ran down a flight of stairs where he confronted and shot a security officer.

It is believed that Alexis took the officer's handgun and returned to the overlook where he continued to shoot. At some point, the official said, Alexis again left the overlook and confronted a victim described as a maintenance person or building staffer. Alexis allegedly shot that person and returned one last time to the overlook where he was ultimately killed in a confrontation with police.

"It didn't appear that he had any plan for escape,'' the official said.

Contrary to earlier reports provided by law enforcement officials, Alexis was not believed to be in possession of an AR-15 rifle.

"No one believes he was looking for anybody in particular,'' the official said.

A witness, Rick Mason, a program management analyst, said the gunman was aiming down at people in the building's cafeteria on the first floor.

Patricia Ward, a logistics management specialist, said she was in the cafeteria. "I heard three shots — pow, pow, pow. Thirty seconds later I heard four more shots."

Then panic, as people tried to get out of the cafeteria. "A lot of people were just panicking. There were no screams or anything because we were in shock."

The federal law enforcement official said it was believed earlier that Alexis may have shot his way into the building because of the location of two victims just outside the building. But it is now believed the victims may have moved there from another location, supporting the theory that Alexis walked into the building without incident and assembled the shotgun inside.

The Metropolitan Police Department identified five additional victims Tuesday morning. They are Arthur Daniels, 51; Mary Francis Knight, 51; Gerald L. Read, 58; Martin Bodrog, 54 and Richard Michael Ridgell, 52.

The seven victims identified Monday night are Michael Arnold, 59; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Kenneth Bernard Proctor, 46; and Vishnu Pandit, 61.

None of the victims have been identified as active-duty military personnel, officials said.

At least three people, including a city police officer, suffered non-fatal gunshot wounds inside Building 197. Hospital officials said all three were expected to recover. Authorities said five other people suffered minor non-gun injuries.

The shock of the rampage, which took place less than three miles from the White House, hung over the city Tuesday

Defense Secretary Chuck Hagel and other officials did not offer any comments as they laid a wreath Tuesday in honor of the victims, but the setting and the somber mood said it all.

As a service member played "Taps," Hagel, along with Chairman of the Joint Chiefs Martin Dempsey and Washington Mayor Vincent Gray, placed the wreath next to "The Lone Sailor" statue that represents "all people who have ever served, are serving now, or are yet to serve in the United States Navy."

U.S. flags were lowered to half-staff at the Congress and White House.

Kevin Johnson reported from Washington. Contributing: Associated Press