The gunman and Queens native involved in Monday's deadly shooting rampage at the Washington Navy Yard reportedly was suffering from serious mental issues, and now FBI investigators have paid a visit to his parents' Brooklyn home to search for more possible clues.
Law enforcement officials say Aaron Alexis, 34, was being treated by the Veterans Administration for issues including paranoia and a sleep disorder.
They also say he claimed to hear voices in his head.
But the Navy had not declared him mentally unfit, which would have rescinded a security clearance that he had from his earlier time in the Navy Reserves.
Investigators say the New York City native used a valid ID to get inside the high security Navy Yard, which is less than two miles from the U.S. Capitol.
Authorities say Alexis acted alone, and was armed with three weapons including an AR-15 assault rifle.
Alexis was killed in a gun battle with police after he opened fire, killing 12 people.
Eight other people were injured, including a police officer, but all are expected to survive.
Alexis was born in Queens and his parents live in Brooklyn.
Police guarded their Bedford-Stuyvesant home last night, shutting down the block as the FBI searched for evidence.
His father told police his son was a first responder after September 11, 2001, but he also suffered from anger issues and post traumatic stress disorder.
Alexis was arrested in 2004 for allegedly shooting the tires of a car over a parking dispute in Seattle.
He was also arrested in 2010 on suspicion of discharging a weapon.
Some of his former neighbors in Flushing say they never would have expected him to be a killer.
"Nothing about him stood out or anything to indicate anything like that. Just a quiet, typical teenager," said one neighbor.
Alexis served in the Navy reserves from 2007 to 2011.
Meantime, flags at the White House and across the country flew at half staff Tuesday morning.
U.S. Secretary of Defense Chuck Hagel also laid a wreath at the U.S. Navy Memorial plaza to honor the victims of Monday's shooting.