Police blocked off roads around the Pentagon on Friday and searched the surrounding area after arresting a man with a suspicious backpack in nearby Arlington National Cemetery.
Police later identified the suspect as 22-year-old Marine Corps reservist Yonathan Melaku.
“Melaku was in possession of a backpack that contained unknown materials which initially caused public safety concerns,” the FBI said in a statement.
“Bomb experts at the scene, however, were able to determine that the items were non-explosive and inert. The materials in the backpack will undergo further testing,” the statement read.
A bomb squad also searched a nearby vehicle, a red Nissan, but found no explosives.
Police closed off several roads around the Pentagon, snarling the morning commute from the northern Virginia suburbs into the US capital. The roads were reopened later in the day.
The US Park Police, which patrols national parks and monuments in the area, said it arrested Melaku overnight in the Arlington National Cemetery.
Park Police spokesman David Schlosser told reporters the heightened security measures were taken after the suspect was “not very cooperative with the answers to what his activities were.”
Melaku, who lives in Alexandria, Virginia — just outside the US capital — is listed as a Marine Corps reservist who has been awarded the National Defense Service Medal and Selected Marine Corps Reserve Medal, according to the FBI.
The FBI and local police also searched Melaku’s home in Alexandria.
US authorities have been on alert for revenge attacks following the killing of Al-Qaeda chief Osama bin Laden in a US raid in Pakistan last month.
Melaku remains in the custody of the United States Park Police.