Officials said they noticed the man, identified by ABC News as 22-year-old Yonathan Melaku of Alexandria, in Arlington National Cemetery after it was closed. They took him into custody and interviewed him.
The questioning led authorities to search for a red 2011 Nissan, which was located in bushes near the Pentagon's north parking lot. Nothing suspicious was found in the car.
ABC News reports that Melaku's backpack contained what officials believe is ammonium nitrate. Bomb experts at the scene determined that the items in the backpack were nonexplosive and inert.
The backpack also reportedly contained notes that mention al Qaeda and the Taliban.
Brenda Heck, special agent in charge of counterterrorism for the FBI, said authorities believe Melaku acted alone. He was arrested and jailed earlier this month for tampering with vehicles in Leesburg.
New York City Police Commissioner Ray Kelly said during a news conference that Melaku is a naturalized citizen and lance corporal in the Marine Corps Reserve, 4th engineer battallion out of Baltimore, Md. He joined the Marines in 2007 and had previously been awarded the National Defense Service Medal and the Selected Marine Corps Reserve Medal.
No charges have been filed yet against Melaku in the latest incident.
Nearby roads were shut down for the investigation, which snarled Washington's morning rush hour.