Aaron Alexis, believed to be the lone gunman in the killing of 12 people at a Navy building in Washington Monday morning, was a Navy veteran from New York with a troubled past who was arrested in two previous shooting incidents.

Alexis, 34, who was discharged from the Navy two years ago after serving hitches in Texas and Illinois, sprayed bullets from the fourth floor down to the cafeteria area in the Naval Sea Systems Command headquarters in southeast Washington in an attack that began just after 8 a.m. Alexis died later as he traded shots with responding police, though it was not clear if he killed himself or was brought down by cops.

At a late night news conference, Metropolitan Police Chief Cathy Lanier said she was confident that Alexis was the "single and sole person responsible" despite earlier reports of a possible second shooter.

Washington Mayor Vincent Gray said that there was still no motive for the shootings and no indication of terrorism "although we haven't ruled that out."

Hewlett Packard issued a statement Monday night saying that at the time of the shootings, Alexis worked for The Experts, a subcontractor on an HP Enterprise Services contract to refresh equipment used on the Navy Marine Corps Intranet network. The FBI said he had legitimate access to the Navy Yard "as a result of his work as a contractor."

Gray said those killed ranged in age from 46 to 73. In addition, he said eight people were injured -- three by gunshots and the other five by non-shooting causes like stress and falls.

Late Monday night, D.C. police identified seven of the 12 victims: Michael Arnold, 59; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Kenneth Bernard Proctor, 46; and Vishnu Pandit, 61.

Two federal officials told the Associated Press that Alexis had an AR-15 assault rifle, a shotgun and a handgun that he took from a police officer at the scene. The two officials spoke on the condition of anonymity because they were not authorized to discuss a pending investigation.

Many who managed to escape in the early minutes of the episode recalled panic and fear after a routine Monday morning was shattered by gunfire.

"They sounded like 'pop, pop, pop,'" said Patricia Ward, a logistics management specialist who was in the cafeteria. "Everybody just panicked at first ... It was just people running, running, running.

"I just kept running," Ward said. "Our mission is to take care of the Navy ... After today, it's not secure enough for me."

Other witnesses told similar stories of gunshots and chaos.

"A little after 8, we heard a loud noise and didn't think anything of it," U.S. Navy Capt. Mark Vandroff told Fox News. "Someone with the presence of mind, and if I find out who it is, I'm going to thank them for it, closed our office doors.

"We got down on the floor as low as we could and barricaded ourselves in with tables and chairs," he added. Vandroff said he frantically texted co-workers to check on them until police led them out of the office around 10 a.m.

Throughout the morning and early afternoon, helicopters circled above the sprawling complex. People still trapped inside the building, where some 3,000 work every day, were told to "shelter in place," as helicopters dropped baskets to pick up the wounded.

Alexis, 34, of New York, was a Navy aviation electrician's mate, third class, who spent time in Texas and Illinois before leaving the Navy in early 2011. It was not clear why he left or if he was voluntarily discharged. A senior defense official told Fox News he had a "history of misconduct."

While stationed in Fort Worth, he was arrested on Sept. 4, 2010, after an upstairs neighbor reported that he had fired shots up into her apartment.The neighbor said Alexis had often angrily confronted her about making too much noise, and she told police he scared her.

"[The neighbor] told me that she is terrified of Aaron and feels that this was done intentionally," the arresting officer wrote on the police report.

In 2004 he was arrested in Seattle for shooting out the tires of a construction worker's car in an incident he later described as "an anger-fueled blackout."

He also told Seattle police he had witnessed the Sep. 11, 2001 terrorist attack and "how these events had disturbed him," according to a police report.

Alexis' father told police in 2004 his son had participated in rescue efforts during 9/11 and had suffered post traumatic stress disorder. This could not be independently confirmed.

A former neighbor of Alexis' family in Queens, N.Y. told Fox News, "I'm shocked and appalled. My heart goes out to those families. I guess you never know who lives next to you."

Lanier said the first call came in just after 8:15 a.m., and that officers were on the scene within minutes. She said one member of her department was shot after engaging with a shooter, who she said was killed. Lanier initially said there were "potentially two other shooters" at large, although one white male who was being sought was later cleared.

The incident paralyzed much of the city, with several nearby schools going on lockdown. The Washington Nationals, who play near the scene, postponed their night baseball game.

Gray said investigators had not identified a motive for the shooting. There was no indication that the incident was an act of terrorism, he said. The FBI was asking the public to provide any information that might help them learn more about Alexis's possible motive by calling (800) CALL-FBI or by clicking here.

Rick Mason, a program management analyst who is a civilian with the U.S. Navy, said a gunman was shooting from a fourth floor overlook in the hallway outside his office. Mason said the gunman was aiming down at people in the building's cafeteria on the first floor, adding that he could hear the shots but could not see a gunman.

Shortly after the gunfire, Mason said overhead speakers told workers to seek shelter and later to head for the gates at the complex.

People who were inside the building said a gunman wordlessly sprayed bullets as terrified civilians and Navy members scattered.

"We saw him hold the rifle, and we saw him aim it in our direction," a witness told FoxNews.com.

Another witness told WJLA: "We were looking, but he was down the hall far enough that we couldn't see a face. But we saw him hold the rifle and then we saw him raise it and aim in our direction."

U.S. Navy Secretary Ray Mabus said two Navy employees and one Metro DC officer were brought to Medstar Washington Hospital Center, not with injuries considered life threatening.

Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget. It builds, buys and maintains the Navy's ships and submarines and their combat systems.

White House officials, in a statement released shortly after the shooting, confirmed that President Obama had been briefed on the incident.

"The President directed his team to stay in touch with our federal partners, including the Navy and FBI, as well as the local officials," the statement read. "We urge citizens to listen to the authorities and follow directions from the first responders on site."

Obama later promised to make sure those responsible for the "cowardly act" are held accountable.

More than 20 members of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) responded to the scene, including the same Special Response Team that extracted the alleged Boston Marathon bomber Dzhokhar Tsarnaev from the boat where he barricaded himself following the April 15 attack.

Although the motive was not yet known, the shooting brought to mind another mass shooting at a military base, on Nov. 5, 2009. That shooting, carried out by Army Maj. Nidal Hasan, an American-born Muslim who became a jihadist while in the military, left 13 dead and more than 30 injured. Hasan was convicted of multiple counts of murder on Aug.23 and sentenced to death by a military judge five days later.

The Associated Press contributed to this report.