According to Flagstaff Police, a bomb team was deployed and the device was detonated.
Flagstaff Police, the FBI and U.S. Postal inspectors are investigating the matter, which has already produced leads.
Chief Deputy Jerry Sheridan said investigators believe the package was picked up on Thursday at a rural post office box. A postal worker at the Flagstaff office believed the package looked suspicious, and a test for explosive residue confirmed the package contained black powder, said Tom Mangan, a spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives.
No one was injured. The investigation continues.