FBI agents are checking "fruitful leads" in a bombing attempt at a Martin Luther King Jr. Day parade where a backpack filled with sophisticated explosives was placed near the marchers' route.

The bomb had a remote detonator and the ability to cause mass casualties, according to an official familiar with the case who spoke on condition of anonymity because he was not authorized to release information about the investigation.

The bomb was defused without incident Monday but unnerved residents of Spokane, especially those who took part in a parade whose theme was steeped in peace and nonviolence.