WASHINGTON (AP) — The deadly attack at the Washington Navy Yard was carried out by one of the military's own: a defense contract employee and former Navy reservist who used a valid pass to get onto the installation and started firing inside a building, killing 12 people before he was slain in a gun battle with police. The shooting was the deadliest on a military installation in the U.S. since the tragedy at Fort Hood, Texas, in 2009. Law enforcement officials say gunman Aaron Alexis carried three weapons at the Navy Yard: an AR-15 assault rifle, a shotgun and a handgun he took from an officer at the scene. D.C. Police Chief Cathy Lanier says investigators now believe Alexis was acted alone. Authorities had earlier said they were searching for a possible second suspect. Investigators say his motive is a mystery.