No explosives found with man in custody near Pentagon.

A suspicious man with a backpack prompted authorities to temporarily close major highways around the Pentagon Friday, but a search of the backpack turned up no explosives.The man, described as a U.S. resident in his 20s, was taken into custody in the predawn hours after he ran from police in Arlington National Cemetery near the Pentagon. Authorities traced him to a car found abandoned near a Pentagon parking lot.

Hours later, the unnamed man was in police custody but had had not been charged with any crime, and the FBI said nothing suspicious was found in the car.

ABC News reported that the backpack contained a substance initially believed to be the explosive compound ammonium nitrate, as well as spent bullet shells and written material referring to al Qaeda and the Taliban.

However, FBI special agent Brenda Heck told reporters that the substance in the backpack was an unknown inert material but it would require further investigation. She said there was no device in the backpack.

"We believe he did act alone," Heck said.

The incident was being investigated by several agencies including the FBI, Pentagon Police, U.S. Park Police and county authorities. A U.S. official described it as "a law enforcement matter."

The Pentagon has been the scene of a number of security incidents since the September 11 attacks in 2001, including an exchange of gunfire outside the building between a man and security guards in March 2010.

Last December, an unattended blinking package prompted authorities to shut the Pentagon's subway station for 90 minutes during rush hour. It turned out to be a Christmas ornament.