Backpack, T-shirts FBI says were found at bomb scene before MLK Jr. Day Parade in Spokane, Wash., Mon. Jan. 17, 2011.

*A backpack containing a pipe bomb was found in Spokane, Washington, along the route of a Rev. Martin Luther King Jr. parade, the FBI said Tuesday.

The device, capable of inflicting “multiple casualties,” was discovered Monday morning by three parade workers before the event, FBI supervisory agent Frank Harrill told CNN. The city’s explosives disposal unit neutralized the device.

The gray backpack was placed on a bench at the northeast corner of North Washington Street and West Main Avenue in downtown Spokane, the FBI said in a statement.

Agents have leads in the case, but Harrill would not provide details on the device and the investigation, which includes a $20,000 reward for the arrest and conviction of those responsible.

The FBI released photos of the Swiss Army-brand backpack and two T-shirts that were found within. One shirt says “Treasure Island 2009″ and the other reads “Stevens County Relay For Life June 25th-26th 2010.” [See photo above.]

The FBI is asking the public for information on who might have been with the backpack from about 8 a.m. to 9:25 a.m. Monday. It also is asking for photos or videos taken in the area.

The incident forced organizers of the parade to change its route, CNN affiliate KREM said.

Ivan Bush, who has helped organize the King birthday celebration march in Spokane for more than 20 years, told the Spokesman-Review that news of the backpack’s potential was “just painful to see and hear.”