Authorities investigating Monday’s D.C. shooting spree believe the gunman brought a shotgun, legally purchased from a suburban Virginia gun shop, to the Navy Yard and acquired additional weapons at the scene, according to three law enforcement officials.

The officials said Aaron Alexis of Fort Worth, Texas, a 34-year-old discharged Navy engineer, entered the building with a Remington 870 12-gauge shotgun that he’d purchased last week in Lorton, Va., and was later found with two additional weapons, one of them a Glock handgun.

One scenario authorities were investigating was that he obtained the handgun and the other weapon at the scene, possibly from victims. But officials cautioned the evidence analysis was still very fluid Monday night.

A key question that remained to be answered was whether an AR-15 semiautomatic weapon found at the scene was involved in the shooting.

Law enforcement officials said Tuesday there was no evidence Alexis obtained, used or fired an AR-15 but that some of the officers who responded at the scene likely brought that weapon.

Forensic analysis wild determine if any of the shots fired came from those weapons.

One of the painstaking tasks for crime-scene technicians will be reconstructing what rounds were fired by the suspect and which were fired by law enforcement or security personnel.

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