Detectives were investigating what prompted the shooting of FJC Security Services guard Idrissa Camara, who was supposed to have gotten off work at 4 p.m. but agreed to stay on for an extra shift.

The shooter, Kevin Downing, entered through a side entrance of the Manhattan building, which houses an immigration court, a passport processing center and a regional office for the Department of Labor, police said.

Downing was a former federal employee at the Bureau of Labor Statistics, police Deputy Commissioner for Intelligence and Counterterrorism John Miller said. But it was unclear if Downing, of Fort Lee, New Jersey, had worked at the building where the shooting occurred, about a mile and a half from the World Trade Center.

Downing had collected Veterans Affairs benefits, but police were still determining which branch of the armed service he served.

He shot Camara, 53, at close range, then walked through a screening area and headed toward an elevator, where he encountered another employee, and then he shot himself in the head, said James O'Neill, a chief with the New York Police Department. He was pronounced dead at the scene.