FBI agents have ruled out terrorism, but U.S. federal regulators estimate it will take them months to determine what caused an oil refinery explosion that killed 15 and injured more than 100. Investigators from two agencies, the Occupational Safety and Health Administration and the U.S. Chemical Safety Board, arrived Thursday at the 1,200-acre BP plant to start sorting through the debris. Also Thursday, an FBI spokesman in Houston dismissed a statement posted on an Islamic Web site claiming responsibility for the blast. He said there was no indication of foul play.