U.S. authorities say they found suspicious items inside the backpack of a man who was detained overnight near Defense Department headquarters.

A U.S. Park Police spokesman told reporters Friday that the man was discovered inside Arlington National Cemetery, just outside Washington.

After interviewing him, the spokesman said police thought it appropriate to look for a vehicle. He said the man was initially “not very cooperative” in answering questions about his activities.

The spokesman said a car was found near the Pentagon, and that the local fire department sent a bomb disposal unit to investigate.

A Federal Bureau of Investigation counterterrorism agent said officials did not find anything suspicious inside the vehicle.

She said the man, who is in his 20s, was carrying a backpack that contained suspicious items, including a non-explosive, unknown material that will require further investigation. She said officials believe he was acting alone.

Officials closed major roads around the Pentagon during the investigation.