By Donna Leinwand Leger, William M. Welch and John Bacon, USA TODAY
WASHINGTON - A shooting rampage at a U.S. Navy command complex in Washington left at least 13 people dead Monday, including a Navy veteran identified as the gunman, authorities said.
Aaron Alexis, 34, a civilian contractor from Fort Worth, was identified by officials as a shooter killed in a gunbattle with police responding to the morning attack at the Washington Navy Yard. A military official said Alexis had been a Navy reservist on active duty before being discharged for misconduct.
The carnage and desperate efforts to stop the shooting gripped the nation's capital in a tense, day-long drama just blocks from the Capitol. Hours after reporting that Alexis was dead, city officials said they had not entirely ruled out the possibility another shooter was involved, but law enforcement officials said it seemed increasingly unlikely that others were involved.
At least three people, including a city police officer, suffered non-fatal wounds in the gunfire inside building 197 at the Naval Sea Systems Command headquarters. Hospital officials said all three were expected to recover.
Hundreds of workers in the Navy complex were forced to hide in their offices or flee for safety while gunshots echoed from a gunman firing a high-powered weapon into the cafeteria and other parts of the building.
A mile or so away at the Capitol, the Senate temporarily locked down all its offices and buildings. The House of Representatives was not in session and did not suspend office functions.
President Obama said he is mourning "yet another mass shooting" and vowed to ensure that "whoever carried out this cowardly act is held responsible."
PHOTOS: U.S. Navy Yard shooting
A federal law enforcement official said Monday that Alexis, who had been staying at a nearby Residence Inn since late August or early September, legally purchased at least some of the weapons used in the assault within the past few days in Virginia.
Alexis allegedly drove to the Navy Yard complex with the weapons early Monday and cleared security checkpoints before parking in a lot on the property, said the official, who was not authorized to comment publicly. After leaving his car, it is believed that Alexis was involved in two altercations in which he opened fire, killing one or possibly two people.
The official said Alexis then entered the building and went to the third and fourth floors, where much of the assault was carried out. He said Alexis did not appear to have an escape plan and it wasn't clear whether he was targeting specific people.
Mayor Vincent Gray said the shootings did not appear to be terrorism-related but said the possibility had not been ruled out.
The Washington Nationals baseball team, which plays home games at a stadium close to the shooting scene, canceled the evening game. At Ronald Reagan Washington National Airport, departures were halted briefly.
Helicopters filled the skies around the Navy complex on the Anacostia River in the Southeast quadrant of the city, an area that has seen a development revival in recent years. Some of the copters airlifted the injured away in baskets suspended beneath the aircraft.
Alexis was an online student at the Fort Worth campus of Embry-Riddle Aeronautical University pursuing a bachelor's of science in aeronautics, the school said.
With the city on edge, the Secret Service arrested a man for tossing firecrackers over the White House fence late Monday. The Secret Service locked down the White House when the incident happened, fearing the pops could have been gunshots.
Gary Humes, a programs manager with the Navy, was entering the building where the shootings took place around 8:20 a.m. when he was met by people fleeing and warning of a shooter inside. He and more than 100 others ran to a building across the street, while others ran to the Navy museum nearby.
"I decided to go in to work a little late this morning,'' he said. "I guess God was with me."
Washington Metropolitan Police Chief Cathy Lanier said one shooter was killed in an exchange of gunfire with authorities and one police officer was wounded. Federal officials identified the dead shooter as Alexis.
Internal security at the Navy Yard building had already "identified and engaged the shooter" by the time the first D.C. police arrived, Lanier said.
She said police exchanged gunfire with the shooter "multiple times" before the final gun battle.
"It's one of the worst things we've seen in Washington, D.C.,'' Lanier said.
Lanier earlier said authorities had information indicating there could have been more shooters. One was later cleared, but police still were searching for a man wearing a military-style uniform and carrying a long gun, she said.
Lanier said the FBI was taking the lead in the investigation.
A federal law enforcement official told USA TODAY that Alexis was armed with an AR-15, which is a light-weight semi-automatic rifle, as well as a shotgun and a handgun.The federal official, who requested anonymity because of the fluid nature of the investigation, said there is no firm evidence that anyone else fired weapons in the attack.
The official said surveillance video of the shooting was being reviewed and scores of investigators were interviewing hundreds of witnesses.
Alexis may have gained entry into the Navy Yard by using someone else's identification card, said a federal law enforcement official who spoke to the Associated Press on condition of anonymity because the official was not authorized to speak publicly.
Terrie Durham, an executive assistant at Naval Sea Systems Command, said a fire alarm sounded and she was trying to leave with a group of people when they encountered a shooter.
"We couldn't see his face, but we could see him with the rifle," Durham said. "He raised and aimed at us and fired. And he hit high on the wall."
Rick Mason, a program management analyst, said a gunman began shooting from a fourth-floor overlook in the hallway outside his office. He 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."
Dave Sarr, an environmental engineer, was walking down a nearby street when he saw people running from the Navy Yard. Sarr has seen an evacuation drill a few days earlier at the Navy Yard. "At first I thought it was another drill," Sarr said. "Then I saw an officer with his weapon drawn."
At MedStar Washington Hospital Center, chief medical officer Janis Orlowski said the hospital was treating three victims - a male D.C. police officer and two women.
She said the police officer had multiple gunshot wounds to his legs and was in surgery. One woman was shot in the shoulder, and the other in the head and hand. All are expected to survive, she said.
Bryan Lynn Chaney, who said he was employed at the Navy Yard through the Wounded Warrior Project, was on the second floor of building 197 when he heard the gunshots.
"I was coming in the main entrance and as I was going up to my office area, I heard what I thought was a locker falling to the ground or slamming a door," Chaney said.
"After that maybe 10 or 15 seconds I heard another couple of bursts,'' he added. "It was confusion. We just knew there was something going on that was unexpected, so we tried to escape out of the area.''
The Navy Yard shootings are the latest in a long line of multiple slayings involving powerful semi-automatic weapons to shock the nation, including the deaths of 20 children and six educators at an elementary school in Newtown, Conn., last December.
Newtown resident Chris Aug called the rampage "another horrible example of our world gone wrong.''
"Instead of healing, the town's wound has been reopened,'' Aug said. "Hopes of optimism are replaced with grief."
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.
NAVSEA headquarters' security requires guests to pass through turnstiles that are watched by security guards before entering. Visitors must also turn in their phones and other electronic recording devices upon entry.
Marine Barracks Washington also put its base on a partial lockdown, only allowing Marines to leave if they were on official business, said Capt. Jack Norton, a base spokesman. A small contingent from Marine Barracks Washington's Guard Company serves at the Navy Yard, Norton said.
Contributing: Marisol Bello; Anne Willette, Kevin Johnson, Jim Michaels, Will Cummings, David Jackson, USA TODAY; Navy Times; WUSA TV; Associated Press