The man was caught after a foot chase and may be mentally ill, a source said, adding that the chemicals that he carried were found not to be dangerous.
The suspect refused to give his name, and authorities were searching for other backpacks in the vicinity.
Police also tracked down the man’s vehicle in bushes near the Pentagon’s north parking lot and called in a bomb disposal team to investigate.
Officials could not confirm a suspicious device was found inside the vehicle, but one or two other people may also have been involved, according to Pentagon police spokesman Chris Layman.
The man was first spotted inside Arlington Cemetery, which is closed to the public overnight, by Department of Defense police who alerted park police.
Inside his backpack were four to five pounds of chemicals that cops thought could be ammonium nitrate, but a preliminary “burn test” came back negative.
Source: Christina Boyle for The NY Daily News.