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(NECN: Alysha Palumbo) - As investigators continue to look into Aaron Alexis' past, including his medical and criminal histories, authorities are learning one of the guns he used in the shooting was a shotgun lawfully purchased in Virginia.
At this time, officials say they believe that's the only gun he had on him when he entered the building.
"We believe at this time that the deceased shooter Aaron Alexis acted alone," said Assistant Director in Charge of the FBI Valerie Parlave.
And alone it appears 34-year-old gunman Aaron Alexis killed 12 other people before he was killed in one of several gun battles with police inside the Navy Sea Systems Command building in D.C.'s Navy Yard.
Officials now say the shooting lasted more than half an hour.
"We believe that Mr. Alexis entered building 197 at the Navy Yard with shotgun, we do not have any information at this time that he had an AR-15 in his possession, we also believe Mr. Alexis may have gained access to a handgun once inside the facility and after he began shooting," Parlave said.
"We had officers who heroically went into a building, witnessing multiple casualties and continued to pursue and engage a gunman who was determined to kill as many people as possible," Metropolitan Police Chief Cathy Lanier said.
Officials say the Fort Worth, Texas-based information technology employee for a defense contractor had been staying in hotels in the D.C. area since August 25, 2013.
Even though Alexis was undergoing mental health treatment by Veterans Affairs for paranoia, a sleep disorder, and hearing voices, and he had been discharged from the Navy Reserves in 2011 after insubordination and disorderly conduct, officials say that early discharge was honorable and his security clearance had not been rescinded.
"Mr. Alexis had legitimate access to the Navy Yard as result of his work as a contractor and he utilized a valid pass to gain entry to the building," Parlave said.
As evidence collection continues on the base, the focus now turns to how and why this tragedy happened.
"We're not going to stop until we get answers to those questions, that's important not only for this city and this community but most of all for the loved ones who were lost yesterday," District of Columbia U.S. Attorney Ronald Machen said.