Driver rams car into FBI building gate, leaves US flag

A driver has rammed a car into a security gate at the FBI building in Pittsburgh, then removed a United States flag from the back seat and threw it over the gate before leaving, authorities say.

The car crashed into the gate about 2.40am, the FBI said, and authorities were searching for the man.

Investigators, including a bomb squad, were at the scene.

No explosives were found, the FBI said.

"We look at this as an act of terror against the FBI," Christopher Giordano, assistant special agent in charge of the FBI in Pittsburgh, told reporters.

"This was a targeted attack on this building. Thankfully no one was hurt but we are going to exhaust every ability we have under the federal law to find, apprehend and prosecute this subject to the fullest extent."

Giordano said he had no information on a motive.

He said the FBI was familiar with the man, a former member of the military.

"He did come here to the FBI field office a few weeks ago to make a complaint that didn't make a whole lot of sense," Giordano said.

Giordano said the car appeared to have some sort of message on one of the side windows but did not elaborate.