SAN JOSE, Calif.—Police and federal investigators sought on Thursday to determine why a Northern California transit employee opened fire on his co-workers, killing nine people in the latest mass shooting to haunt the United States. The accused gunman shot himself as police closed in on him, minutes after gunfire erupted at about 6:30 a.m. Pacific time at a light rail yard in the heart of Silicon Valley, according to Santa Clara County Sheriff Laurie Smith. Police did not publicly name the gunman. The San Jose Mercury News and other news media identified him as Samuel Cassidy, 57, a maintenance worker at the yard. Firefighters responded to a fire at a home where the suspect lived at about the same time that the shooting was first reported. A police bomb squad searched the rail yard and adjacent buildings after an explosive device was found. Police declined to speculate on a motive …