The gunman who killed 12 people at a Navy base in Washington entered with a valid pass and a legally purchased shotgun, the FBI said Tuesday. Once inside, Aaron Alexis, 34, may have obtained a handgun after he began shooting, said Valerie Parlave, assistant director of the FBI's Washington field office. He brought the shotgun legally in neighboring Virginia, and there is no evidence he had an assault rifle as some reports have said, Parlave addded. Alexis, who was shot and killed after his rampage Monday, was working on a Navy contract through computer giant Hewlett-Packard. "Mr. Alexis had legitimate access to the Navy Yard as a result of his work as a contractor, and he utilized a valid pass to gain entry to the building," Parlave told a news conference. His motive remains a mystery, she said. "We continue to conduct interviews, exploit digital media, and run down every lead we can to piece together his recent movements and determine the motive behind his attack," she said. Alexis arrived in the Washington area on or about August 25th and stayed in hotels, she said. The FBI has received hundreds of tips from the public, she added