A backpack found along the route of the Martin Luther King Jr. march in Spokane contained a bomb "capable of inflicting multiple casualties," the FBI said Tuesday, describing the case as "domestic terrorism."
The FBI said the Swiss Army-brand backpack was found about 9:25 a.m. PST on Monday on a bench at the northeast corner of North Washington Street and West Main Avenue in downtown Spokane.
In an interview on msnbc cable's The Rachel Maddow Show, Spokesman-Review reporter Thomas Clouse said confidential sources told him that the device was equipped with a remote control detonator and contained shrapnel.
A bomb disposal unit was called in and neutralized the device with a robot. The FBI said in a statement on Tuesday that "the backpack contained a potentially deadly destructive device, likely capable of inflicting multiple casualties."
The FBI has refused to discuss how the bomb was constructed.
"Suffice it to say it was of grave concern," Frank Harrill, special agent in the charge of the Spokane FBI office, told NBC News.
"You could describe it as an improvised destructive device ... or improvised explosive device."
The FBI has not established an official motive, but Harrill told NBC News "the timing and placement of the backpack (along the march route) is inescapable."
"At that point, it falls directly in the realm and sphere of domestic terrorism," Harrill told the Associated Press. "Clearly, there was some political or social agenda here."
No threats or warnings were issued before the march.
Workers initially reported the suspicious package to Spokane police. The march was slightly re-routed and delayed.
"The three contract workers in the area who were there are unsung heroes," the FBI spokesman said.
"We asked the FBI if we should be concerned, and he said it was some sort of device that was a real explosive device and something to be concerned about," Melissa Opel, who works at Auntie's Bookstore near the scene, told NBC station KHQ of Spokane.
Investigators were hoping two T-shirts found inside the backpack will offer clues to suspects in the case. One reads "Stevens County Relay For Life June 25th-26th 2010" and another shirt reads "Treasure Island Spring 2009," the Spokesman-Review reported.
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.
The FBI said it was seeking photographs or video taken in the area from approximately 8 a.m. to 11 a.m. People with information were asked to contact the FBI: 206-622-0460 or email firstname.lastname@example.org.
Another explosive device was found March 23 beside the Thomas S. Foley U.S. Courthouse in downtown Spokane. No arrests have been made in that investigation, Harrill said, and agents didn't know if the two incidents were related.
The Spokane region and adjacent northern Idaho have had numerous incidents of anti-government and white supremacist activity during the past three decades.
The most visible was by Aryan Nations, whose leader Richard Butler gathered racists and anti-Semites at his compound for two decades. Butler was bankrupted and lost the compound in a civil lawsuit in 2000 and died in 2004.
In December, a man in Hayden, Idaho, built a snowman on his front lawn shaped like a member of the Ku Klux Klan holding a noose. The man knocked the pointy-headed snowman down after getting a visit from sheriff's deputies.
NBC News' Jonathan Dienst, msnbc.com staff and The Associated Press contributed to this report.