The man was discovered inside Arlington National Cemetery after 1 a.m., several hours after the cemetery had closed, and was taken into custody after being uncooperative. He was in custody but has not been charged with anything. Authorities believe he acted alone and no one else was with him.
The man was identified as Yonathan Melaku, 22, a naturalized U.S. citizen originally from Ethiopia, according to a law enforcement official who spoke on the condition of anonymity. Investigators and bomb-sniffing dogs were sifting through his family's home in Fairfax County, Va.
The man had an unknown quantity of a substance that appeared to be ammonium nitrate in a bag, according to another law enforcement official speaking on the condition of anonymity. Ammonium nitrate is a chemical compound that is widely used in fertilizers and can be used in explosives with the correct concentration.
Nothing else was found on the man that could have triggered an explosion, the official said.
The officials spoke on the condition of anonymity because they were not authorized to release the information. Tests were being done to determine the substance and the exact concentration, the second official said.
During the course of the investigation, officers searched the man's nearby car, a red 2011 Nissan that was parked in the bushes near a Pentagon parking lot, but found nothing suspicious inside, said Brenda Heck, special agent in charge of the counterterrorism unit of the FBI's Washington field office.
She would not disclose the materials inside the backpack, but said it contained no explosives.
In another incident earlier this week, a motorist found with a gun and what appeared to be a suspicious package near the Pentagon was taken into custody.
Friday's investigation snarled rush hour traffic as police closed off roads around the Pentagon.
Sullivan reported from Washington. Associated Press writers Lolita Baldor, Jessica Gresko and Karen Mahabir contributed to this report.