DEVELOPING: Authorities are investigating a man found overnight inside Arlington National Cemetery with a backpack containing "suspicious items and products," the FBI said Friday. But officials say the backpack does not contain explosive material and is undergoing further testing.

FBI special agent Brenda Heck said during a press conference Friday that the man, whom she did not identify, was found inside Arlington Cemetery overnight when the cemetery was closed.

Heck said the man, described as in his 20's, had a backpack containing a "non-explosive, unknown material" that she said will require further investigation.

"The products in the backpack are inert," Heck said, though she described its contents as suspicious.

Though the man was "not forthcoming" when taken into custody, Heck said authorities later found his red 2011 Nissan near the Pentagon.

She said authorities found "no suspicious activity" related to the vehicle.

Heck would not confirm earlier reports that a notebook was also found containing threatening phrases like "Al Qaeda" and "Taliban rule" and "defeated coalition forces."

Heck went on to say that the man is believed to have acted alone and remains in custody. No charges have been filed against him, she said. reported that the man in custody is a naturalized U.S. citizen who came to the country from Ethiopia.