Update (12:20 p.m. EDT): A story by CBS has word that the suspect under arrest is Lance Cpl. Yonathan Melaku a reservist in the U.S. Marine Corps. Melaku reportedly told authorities he was carrying an explosive in his backpack. 

 

Previously, FBI Special Agent Brenda Heck, who heads the bureau's counterterrorism division in its Washington field office, told reporters that a non-explosive material was found in a backpack the suspect was carrying at the time of his arrest.

A law enforcement official speaking on the condition of anonymity said officials found what appeared to be an unknown quantity of ammonium nitrate. The official, who was not authorized to release the information, said nothing else was found that would have enabled an explosion. The official said tests were being done to determine the substance and the exact concentration.

Ammonium nitrate is a chemical compound that is widely used in fertilizers and can be used in explosives with the correct concentration.

Despite the fact that Melaku was carrying written materials advocating al Qaeda and the Taliban, investigators are not considering this morning's arrest to be terrorism-related.

Original story: The drama at the Pentagon this morning happened outside its walls, near the site of the Sept. 11, 2001 attack, where police today arrested a man "carrying a suspicious device and pro-al Qaeda literature," ABC reported. Police are reportedly looking for two others "who 'fled the scene' on foot," NBC reported. We still don't know much about the man himself, except that he "appears to be a U.S. citizen of Ethiopian ethnicity." But the contents of his car were enough for police to take him into custody.

"Sources told ABC News the man's backpack contained what officials suspect is ammonium nitrate and spent 9mm shells as well as written material that contained statements including: 'al qaeda taliban rules.' " According to NBC, U.S. Park Police first became suspicious when they found the man in Arlington National Cemetery after hours. He was "uncooperative" when they interviewed him, which led police to search his vehicle, where they found the al Qaeda literature. The incident shut down traffic around the Pentagon, which will surely disrupt the workday for many Washington, D.C. commuters.