Melaku fled, but the police ran him down and found four zip-lock bags with a substance labeled ammonium nitrate, a key bomb-making material, in his backpack, officials said. Also inside were spent 9mm ammunition and a notebook containing the words "al qaeda," "Taliban rules," "mujahidin" and "defeated coalition forces," according to police.
Police worried that Melaku had planted bombs at the cemetery and the Iwo Jima memorial located just a mile from the Pentagon. All traffic was shut down around the Pentagon while authorities raced to Melaku's suburban home in Alexandria, Va., to search for bomb making material.
Searches of Melaku's home and car turned up no explosives, and the material in Melaku's backpack tested negative as a potential explosive, sources said.
Melaku was arrested last month for smashing windows and stealing valuables from 27 cars in Leesburg, Va., and charged with four counts of grand larceny.
A man by the same name and birth month was also arrested in Fairfax County for reckless driving and failure to stop. He pleaded guilty and paid a $200 fine for the former charge and $30 for the second charge.
Melaku, who joined the Marine Corps Reserve on Sept. 4, 2007, according to the FBI, had been awarded the National Defense Service Medal and the Selected Marine Corps Reserve medal.
ABC News' Pierre Thomas and Bianna Golodryga contributed to this report.