(CBS/AP) - The FBI believes deceased Navy Yard shooting suspect Aaron Alexis acted alone and therefore the investigation has moved into a phase of "evidence recovery and information gathering," Valerie Parlave, Assistant Director in charge of the Washington Field Office said in a press conference Tuesday afternoon.
Parlave said the remainder of the investigation is a "methodical and intensive process that includes bullet trajectory and crime scene mapping."
Parlave confirmed that Alexis entered building 197 at the U.S Navy Yard with a shotgun purchased legally in Virginia and said the FBI believes he may have gained access to one or more handguns once inside the building. She said they do not have any information that Alexis had an AR-15 assault rifle in his possession, contrary to some initial reports.
Police say Alexis, a 34-year-old defense contractor employee, used his security clearance pass to get into the Washington Navy Yard Monday before spraying bullets into hallways and shooting from a balcony onto workers in an atrium below, killing a dozen people and injuring eight more. Out of the eight hurt, three were shot, including a Metropolitan Police officer who suffered gunshot wounds to his legs and two female Navy Yard employees. They are all expected to survive.
Alexis was killed in a gun battle with police.
Parlave said Alexis had arrived in the D.C. area on or about Aug. 25 and that he had been staying in hotels in the area since then. She said the FBI is continuing to work on compiling information about the suspect and she urged anyone with information to notify the FBI.
"No information is too small," she said.
The motive for the mass shooting - the deadliest on a military installation in the U.S. since the attack at Fort Hood, Texas, in 2009 - remains unclear.
U.S. law enforcement officials told The Associated Press that there was no known connection to terrorism and that investigators have found no manifesto or other writings suggesting a political or religious motive.
Alexis had been suffering a host of serious mental problems, including paranoia and a sleep disorder, and had been hearing voices in his head, according to the officials, who spoke on condition of anonymity because the criminal investigation was still going on.
He had been treated since August by Veterans Affairs, the officials said.
The Navy had not declared him mentally unfit, which would have rescinded the security clearance Alexis had from his earlier time in the Navy Reserve.
A Buddhist convert who had also had flare-ups of rage, Alexis, a black man who grew up in New York City and whose last known address was in Fort Worth, Texas, complained about the Navy and being a victim of discrimination. He also had run-ins with the law but was not charged over shootings in 2004 and 2010 in Texas and Seattle, and was ticketed for disorderly conduct after being thrown out of a metro Atlanta nightclub in 2008.
Alexis' bouts of insubordination, disorderly conduct and being absent from work without authorization prompted the Navy to grant him an early - but honorable - discharge in 2011 after nearly four years as a full-time reservist, authorities said. During his service, he repaired aircraft electrical systems in Fort Worth.
At the time of <Monday's rampage, Alexis was an employee with The Experts, a Florida-based company that was a Defense Department subcontractor on a Navy-Marine Corps computer project, authorities said.
Thomas Hoshko, the CEO of The Experts, says Alexis had "no personnel issues" he was aware of. He also says a background check of Alexis as recently as July revealed no criminal history.