At least thirteen people were killed on Monday in a shooting rampage involving one or two gunmen at a naval building only a few miles from the White House.

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Among the dead on the grounds of the capital’s Navy Yard was a suspected gunman, identified by officials as 34 year old Aaron Alexis, a man from Fort Worth, Texas. Reports said he had been found with three weapons, including an AR-15 assault rifle, a semiautomatic pistol and a shotgun.

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Alexis received a general discharge from the Navy Reserve in 2011 after a series of misconduct issues, a Navy official said. The official declined to detail the types of misconduct in Alexis' record.

At least four people were reported to have been wounded in the early morning rampage, the worst mass shooting in the country since the incident at Sandy Hook Elementary School in Newtown, Connecticut, in December. The death toll on Monday night equalled that at the Fort Hood military base in Texas in 2009, although in that incident the gunman, Major Nidal Hasan, was apprehended alive.

Early in the afternoon, the police raised the possibility that the rampage might have involved as many as three gunman, saying they were searching for two potential suspects. But later one of the potential suspects was identified and cleared of any involvement in the shooting, leaving one outstanding person whom the police were still looking for on Monday night. With a potential suspect at large, the US Senate complex went into lock down in the afternoon.

Washington DC police chief Cathy Lanier earlier described the possible suspect as an African-American male, about 5-foot-10 and around 50 years of age. He was believed to be dressed in an olive drab military style uniform and armed with what she described as a “long gun.”

President Obama condemned the shooting, labeling it a “cowardly act.” “So we are confronting yet another mass shooting, [and] today it happened on a military installation in our nation’s capital, affecting both military and civilian personnel,” he said.

According to the Washington Post, the dead suspect, Aaron Alexis, was a native of Brooklyn who until recently was a resident of Texas and had a Navy or military background. Speaking to the paper, his aunt, Helen Weeks, said: “We haven’t seen him for years. I know he was in the military. He served abroad. I think he was doing some kind of computer work.” Ms Weeks said she had not been contacted by the police. Of Aaron’s suspected involvement, she added: “I’d be shocked if it was him, but I don’t know.”

Meanwhile, a medical official at George Washington University Hospital confirmed that a man in his 60s with a gunshot wound to his head had been pronounced dead upon his arrival from the Yard earlier in the day. “His wound was not survivable,” Dr Babak Sarani, the hospital’s director of trauma and acute surgery, told reporters.

At the MedStar Washington Hospital Center, officials said three victims had been brought to the hospital. All three were in critical condition but “their chances for survival are very good,” according to the hospital’s chief medial officer, Janis Orlowski.

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As law enforcement searched for the two other potential shooters, eyewitnesses from the Yard described the rampage. The building at the centre of the rampage is the headquarters of Navy Sea Systems Command (NSSC). In all, about 3,000 people work at the secure facility, known as building 197, including Todd Brundidge.

An executive assistant with the NSSC, Mr Brundidge told the Associated Press that he and his colleagues encountered a gunman dressed in blue in a hallway on the third floor of building. “He just turned and started firing,” he said. Later, speaking to a local TV station, he described how people began rushing out of the building. “Everyone was going down the stairs,” he told WJLA. “

“They were pushing. They were shoving. People were falling down. As we came outside, people were climbing the wall trying to get over the wall to get out of the spaces. It was just crazy.”

Over in the the cafeteria, Patricia Ward, a logistics management specialist, was getting breakfast when she heard the sound of gunfire. “It was three gunshots straight in a row - pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop, so it was like about a total of seven gunshots, and we just started running,” she said later, speaking to reporters near the Navy Yard.

Earlier, Navy officials told the Washington Post that, as the rampage unfolded, they had evacuated Admiral Jonathan Greenert, the chief of US Naval Operations and a member of the Joint Chiefs of Staff, and his wife from the Navy Yard to the Pentagon.

As the afternoon unfolded, the Navy issued an “Order to Account” for all uniformed personnel in the District of Columbia metro area, ordering them to check-in with authorities.

Base shootings: previous attacks

In June 1994, Dean Mellberg, 20, killed four people and wounded 23 at a hospital on Fairchild Air Force Base in Washington state before he was shot dead. The New York Times reported a psychiatrist and a psychologist whose observations had led to Mellberg’s discharge from the air force were among the dead.

Sgt William Kreutzer killed an officer and wounded 18 soldiers at Fort Bragg, North Carolina, as they took part in physical training in October 1995. Kreutzer was initially sentenced to death, but he was later given a life sentence.

Thirteen people were killed and dozens wounded after Major Nidal Malik Hasan, an army psychiatrist, went to a medical facility at the Fort Hood base in Texas and started firing in November 2009. He was sentenced to death last month.

In March this year, Marine Sgt Eusebio Lopez shot and killed two colleagues at an officer candidates school on the Quantico Marine Corps base in Virginia.