A third man has been arrested in connection with the Washington-area sniper attacks, but police say he's not a suspect in the killings.

A spokesperson at the FBI's Philadelphia office said Nathaniel O. Osbourne was arrested Saturday morning in Flint, Mich. He is being held as a material witness.

John Allen Muhammad, 41, and teenager John Leo Malvo were arrested near Frederick, Va., on Thursday.

Police believe Osbourne helped the others buy the car allegedly used in the series of sniper attacks. He is listed as a co-owner of a blue Chevrolet Caprice in which Muhammad and Malvo were arrested.

A killing spree terrorized the Washington area for three weeks, during which 10 people were shot dead and another three were seriously wounded.

Prosecutors in Maryland filed six charges of first-degree murder against the pair on Friday. Others killed in the serial attacks were in Virginia.

Murder charges were also filed in Alabama related to a robbery and killing there earlier this year.

Maryland authorities said they would ask for the death penalty against Muhammad. But while they intend to try Malvo as an adult, the state doesn't allow people under the age of 18 to be executed. Malvo is reported to be 17.

Virginia and Alabama, however, allow the death penalty for crimes committed at the age of 16.