WASHINGTON — A mass shooting Monday morning at the Washington Navy Yard left at least 13 people dead, including a suspected gunman from Fort Worth who allegedly shot his way into the secure military facility and was then killed by police, officials said.It was perhaps the most devastating shooting attack in the heavily guarded nation’s capital in decades and sparked new concerns about security at U.S. military facilities.Parts of the nation’s capital were put on lockdown after the shooting on the edge of Capitol Hill as the police sought two other armed suspects spotted by video cameras. But by Monday evening, federal authorities said they believed the shooting was the act of a lone gunman.District of Columbia Police Chief Cathy Lanier identified the gunman as a civilian Navy contractor, Aaron Alexis, 34, of Fort Worth who was identified by fingerprints. An employee of a Hewlett-Packard subsidiary, Alexis was a full-time Navy reservist until 2011, last serving with a logistics support squadron in Fort Worth.There were unconfirmed reports that he used a friend’s identification to get into a building at the Washington Navy Yard before he sprayed bullets on prone victims until police arrived, exchanging gunfire with him several times before he was shot dead.By nightfall, authorities were still seeking to question a black man between 40 and 50 years of age described as wearing an olive-colored military-style uniform; it was unclear whether he was involved in the shooting, or whether Alexis was a lone gunman. Lanier said a potential third suspect was cleared of any involvement.Alexis had a police record for gun-related incidents in the states of Texas and Washington, according to legal documents and law enforcement.The FBI posted a “Seeking Information” notice about Alexis, showing his photograph and asking for help in piecing together how and why he was in the nation’s capital.“We don’t have any reason at this stage to suspect terrorism,” said D.C. Mayor Vincent Gray, though he said it hasn’t been ruled out.A second-floor cafeteria off the atrium of the Naval Sea Systems Command headquarters, a workplace for 3,000 people, was bustling with morning traffic when one or more gunmen opened fire around 8:20 a.m. EDT from a fourth-floor mezzanine, the Navy said. The wounded survivors described hearing a steady “pop-pop-pop,” but it was unclear whether the shooter had a semiautomatic weapon, said Dr. Janis Orlowski, the chief medical officer at the Medstar Washington Hospital Center.Gary Humes, whose LinkedIn profile lists him as director of strategic operations for the Navy, told reporters that while inside the locked-down building, people searched their computers upon learning Alexis had been identified as the shooter. They found him listed as a computer specialist under contract in San Diego.Chaotic sceneNavy contractor Sean Carroll described to McClatchy a chaotic scene once the shooting started.“People didn’t realize what you were supposed to do,” he said. “Just heard the sounds. It was really loud. You could hear the gunshots. That’s a surreal thing. You’re not really thinking. But it wasn’t like, ‘Hey what’s going on?’ You know with the world we live in. You know, ‘Oh my God. This is Fort Hood.’”Fort Hood in Texas was the scene in 2009 of a shooting by a member of the military that left 13 dead and more than 30 injured.Word that Alexis had a prior rap sheet spread in late afternoon. Police records in Fort Worth show that on Sept. 4, 2010, officers were dispatched to his apartment when a neighbor alleged he shot a hole up through his ceiling.Alexis explained that the gun discharged after it slipped while he was cleaning it. He was arrested but the charges were never pursued.Police in Seattle put out a statement saying that Alexis had been arrested on June 3, 2004, for an “anger-fueled blackout” in which he shot out the rear tires of a car at a construction site near his home. Construction workers said he had stared at them angrily for about a month before the shooting. Alexis, said the police statement, told them he didn’t remember the shooting initially.“Detectives later spoke with Alexis’ father, who lived in New York at the time, who told police Alexis had anger management problems associated with post-traumatic stress disorder, and that Alexis had been an active participant in rescue attempts on September 11th, 2001,” the Seattle police statement said.The FBI, which took the lead in the investigation, was trying to learn more about the past of the alleged gunman, whom friends called a practicing Buddhist.“Our [intelligence] teams and our investigative teams are looking at those reports, and trying to come up with a better understanding of who he is and exactly what he was doing,” FBI spokeswoman Lindsay Godwin said.Security guards at the Navy Yard scrambled to engage the gunmen, and police officers arrived within two to three minutes, Lanier said, and “shooter teams,” assembled from several agencies, were on site within seven minutes. After several exchanges of gunfire, the assailant died in a final shootout with metro police, Lanier said.Lanier credited D.C. police and the U.S. Park Police for preventing even more bloodshed in the morning rampage.“I think the actions by the police officers, without question, helped to reduce the numbers of lives lost,” Lanier said, calling the actions of first responders “nothing short of heroic.”“This is a horrific tragedy,” Gray said. “I don’t know if we’ve ever had one like this … nothing like this in the District of Columbia.”Three gunshot victims, including a police officer, were brought to MedStar Washington Hospital Center. In a press conference, Orlowski, the center’s chief medical officer, said their chances of survival were good. She confirmed that all three victims were alert and talking when they arrived.The wounded police officer was shot in both of his legs and underwent long hours of surgery to repair blood vessels and bones, she said. She said doctors would try to assess on Tuesday his chances of being able to walk again.The other two wounded survivors are both female civilians. One suffered wounds to her shoulder and underwent hours of surgery. The other was shot in the head and hand, but luckily the bullet did not penetrate her skull and she didn’t need surgery, Orlowski said.President Barack Obama, who ordered flags at government buildings lowered to half-staff through Friday to honor the victims, vowed that his administration would do “everything in our power to make sure whoever carried out this cowardly act is held responsible.”The neighborhood near the naval facility remained sealed off, with residents locked out and nearby schools locked down. The Senate was placed on a preventive lockdown early Monday afternoon, with votes postponed. The lockdown was later partially lifted. The Washington Nationals, whose stadium is just blocks from the crime scene, postponed their game Monday night against the Atlanta Braves.The area around the Navy Yard, not far from Capitol Hill, had been sealed off by layers of law enforcement personnel from local and federal agencies. Senate Sergeant at Arms Terrance Gainer issued a mid-afternoon statement announcing that no one would be allowed in or out of Senate offices, but later lifted it partially.The shooting began inside the Naval Sea Systems Command Headquarters building, a workplace for 3,000 people, according to a press release from Naval District Washington.Patricia Ward, a logistics management specialist, told reporters that she was near the cafeteria area when she heard some shots, followed by a break and then more shots. She and others fled the building. The gunman was described by witnesses as carrying an AR-15 assault rifle, a double-barreled shotgun and a handgun.An elite team from the Bureau of Alcohol, Tobacco, Firearms and Explosives was dispatched to the scene Monday, part of the same ATF Special Response Team Canine Program that helped capture the suspect in the Boston Marathon bombing. David Lightman, Maria Recio, Greg Gordon, Mary Faddoul, Kendall Helblig, Tish Wells, Lesley Clark, Lindsay Wise and William Douglas contributed to this report, which included material from The New York Times.