SPOKANE, Washington (AP) — An explosive device found inside a backpack along a downtown Martin Luther King Jr. Day parade route was a credible threat capable of causing multiple deaths, the FBI said Tuesday.

The device was successfully disposed of by the FBI’s explosive unit after the backpack was found on a park bench by a Spokane city employee about an hour before the parade was set to begin.

Frank Harrell, special agent in the charge of the Spokane FBI office, wouldn’t discuss what specifically made the bomb so dangerous. But he said the investigation has become a top priority.

“It definitely was, by all early analysis, a viable device that was very lethal and had the potential to inflict multiple casualties,” Harrell told The Spokesman-Review.

Harrell said the timing and placement of the device wasn’t coincidental.

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

The bomb was discovered at 9:25 a.m. in a Swiss Army-brand backpack that also contained two T-shirts.

Another explosive device was found March 23 beside the Thomas S. Foley U.S. Courthouse in the eastern Washington city. No arrests have been made in that investigation, Harrell said, adding that agents didn’t know if the two incidents were related.

The FBI is offering a $20,000 reward for information leading to the arrest and conviction of people responsible for placing the device found Monday.

Harrell said the people who discovered the backpack and brought it to the attention of police were the heroes of the day. Police quickly rerouted the parade and called for the bomb disposal unit, he said.

Copyright 2011 The Associated Press.