A Marine reservist taken into custody under suspicious circumstances at Arlington National Cemetery early Friday sparked an aggressive police response that led to road closures near the Pentagon and massive traffic tie-ups throughout morning rush hour.

Although authorities eventually determined there was no immediate threat to the public, they said they could not afford to take any chances. They noted that the suspect, Yonathan Melaku, 22, initially fled from police, had suspicious items in his backpack and even told officers that the bag contained bombmaking materials.

Soon after Melaku’s apprehension, agents on the FBI’s Joint Terrorism Task Force descended on the scene. Police closed ramps to Interstate 395 and roads around the Pentagon to prevent bystanders from getting too close to Melaku’s car, a red Nissan discovered in a wooded area near the Pentagon.

At one point, the Arlington County bomb squad used a device to smash out the Nissan’s window to access the car. Police said they found no explosives in the backpack or the car.

“We need to always be vigilant in the D.C. area,” said Sgt. David Schlosser, a U.S. Park Police spokesman.

By late morning, officers and agents were searching Melaku’s home in the Alexandria section of Fairfax County. It was not clear what they might have found in the red-brick townhouse.

Authorities said the incident began about 1:30 a.m. when police spotted Melaku in the cemetery, which is closed to visitors overnight, and he ran from them. Melaku was then detained by U.S. Park Police, authorities said, and told officers that his backpack contained bombmaking materials, including ammonium nitrate, a compound used as a fertilizer and as a component of explosives, according to a law enforcement source, who requested anonymity because the investigation is ongoing.

Initial tests on several baggies in the backpack did not appear to reveal traces of the compound, the source said. Authorities said the backpack and its contents were being sent to the FBI’s laboratory for further testing.

Police also discovered a notebook, apparently related to a class Melaku was taking, in the backpack that had the words “Al Qaeda” and “Taliban” written on its pages, the source said.

Schlosser said authorities learned about the Nissan linked to Melaku and closed off roads while experts were brought in to examine the car. The closures snarled morning rush-hour throughout the Arlington area and caused a major back-up on I-395 north from the 14th Street bridge to the Springfield interchange.

U.S. Park Police did not say whether charges would be filed. This was not Melaku’s first brush with law enforcement. Last month, he was arrested in connection with the vandalism of 25 cars in the Leesburg area. His attorney in that case, Robert May, did not respond to phone messages seeking comment.

The FBI reported that Melaku joined the Marine Corps Reserve on Sept. 4, 2007, and has been awarded the National Defense Service Medal and the Selected Marine Corps Reserve Medal. He has not deployed overseas, the FBI said.

Johnny Briley, 54, who lives in Melaku’s townhouse development, said an older man who is a cabdriver lives in the home authorities searched. Briley said he has seen Melaku coming and going at the townhouse. Another neighbor said he has seen Melaku riding a bicycle in the neighborhood.

Briley said that the residents of the townhouse are quiet and that they often have visitors. “A lot of commotion, people going in and out, but they kept to themselves,” he said.

“I hope this is just an extreme overreaction erring on the side of caution,” said a neighbor who declined to give her name. “All the commotion is a bit unnerving, but I’m comfortable that we’re safe.”

Staff writers Caitlin Gibson, Allison Klein, Jerry Markon and Del Quentin Wilber contributed to this report.