The FBI offered a reward Tuesday for information about a potentially lethal bomb found in a backpack along the downtown Spokane route of the Martin Luther King Jr. Day parade.

An abandoned backpack found Monday along the route of Spokane’s annual Martin Luther King Day march contained a bomb capable of inflicting “multiple casualties,” the FBI has confirmed.

The bureau’s terrorism task force is offering a $20,000 reward for information leading to the arrest and conviction of those responsible for planting the bomb.

The FBI on Tuesday issued a bulletin asking for the public’s assistance.

Frank Harrill, special agent in charge of the Spokane office, would not discuss what specifically made the bomb so dangerous but said the investigation has become a top priority.

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“It definitely was, by all early analysis, a viable device that was very lethal and had the potential to inflict multiple casualties,” Harrill said.

“Clearly, the timing and placement of a device — secreted in a backpack — with the Martin Luther King parade is not coincidental. We are doing everything humanly possible to identify the individuals or individual who constructed and placed this device.”

Two security sources, who did not want to be named because they were not authorized to give information, told The Spokesman-Review they received a briefing suggesting the bomb was designed to detonate by a remote device, such as a keyless entry remote for a vehicle or a garage-door opener.

The bomb apparently also had its own shrapnel that could have caused significant injuries to anyone near the blast.

Ivan Bush, who has helped organize the march in Spokane for more than 20 years, said news of the backpack’s potential was “just painful to see and hear.”

“Man, that’s a sad testament,” said Bush, who works for Spokane Public Schools. “Here we are in the 21st century, and these types of things are still happening. It just hurts.”

The purpose of the march was to bring residents together to celebrate a man who championed passive resistance, he said.

“This community came together to get a street dedicated to Dr. King and thousands come out to celebrate him every year,” Bush said. “When something like this takes place, it’s just painful.”

The bomb was discovered in a Swiss Army-brand backpack that was placed on a park bench at the northeast corner of North Washington Street and West Main Avenue.

Two T-shirts were in the bag. One reads “Stevens County Relay For Life June 25th-26th 2010″ and another shirt reads “Treasure Island Spring 2009.” The FBI is working with other federal agencies and virtually all local police agencies with the investigation as part of the Northwest Joint Terrorism Task Force.

“I think the link to the Martin Luther King celebration and march is inescapable,” Harrill said. “At that point, it falls directly in the realm and sphere of domestic terrorism. Clearly, there was some political or social agenda here.”

Harrill said he could not discuss any potential suspects.

Meanwhile, federal investigators continue to investigate an explosive device discovered March 23 alongside the Thomas S. Foley U.S. Courthouse. Harrill said agents have not made an arrest in that previous case.

“We don’t know, at this point, of any linkage to any other incident,” Harrill said, referring to the Monday discovery. “We are not aware of any other events that prefaced this event … or threats associated with this device. Nor does it appear to be linked to any other incidents in Spokane or anywhere else in the country. But, that certainly is a focus for us.”