updated 1/20/2011 12:51:25 PM ET 2011-01-20T17:51:25

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.

Story: FBI: Bomb found on MLK march route

The attempt on the day set aside to honor the slain civil rights leader raised the possibility of a racial motive in a region that has been home to the white supremacist group Aryan Nations.

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"The confluence of the holiday, the march and the device is inescapable, but we are not at the point where we can draw any particular motive," Harrill said.

Investigators have no suspects but are following up on "fruitful leads" they've received from the public, said Frank Harrill, special agent in charge of the Spokane FBI office. He declined to elaborate.

Harrill said the black Swiss Army backpack has been shipped to an FBI lab in Quantico, Virginia, for analysis. In addition to the bomb, the backpack contained two T-shirts that authorities were analyzing for clues.

Heroes
Officials have praised as heroes the three city workers who spotted the backpack about an hour before the parade was to start. Harrill said they looked inside, saw wires and immediately alerted law enforcement.

Video: FBI hunts for suspects in MLK parade bomb scare (on this page)

The bomb was carefully placed on a metal bench with a brick wall behind that would have directed shrapnel toward Main Street, where marchers were expected to pass, investigators said.

No one has claimed responsibility for planting the bomb, Harrill said.

The FBI released a photo of the backpack as it offered a $20,000 reward for information from the public. Also released were pictures of the T-shirts found in the pack. There was a gray T-shirt with writing for the Stevens County Relay for Life race last June. Stevens County is just north of Spokane County. The other dark T-shirt said "Treasure Island Spring 2009."

Investigators are seeking anyone who took photographs or video in the area between 8 a.m. and 11 a.m. Monday.

Monday's parade route was changed after the bomb was found, and most of the several hundred people who marched in the annual event did not know about the device, said Liz Moore, one of the marchers.

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"No announcements were made at all," Moore said. "It seems like a specific effort was made not to alarm people."

Harrill called the bomb an act of domestic terrorism.

Spokane County Commissioner Mark Richard, who spoke at the King celebration and did not learn of the bomb until later, expressed concern about the number of people who could have been injured or killed if it had detonated.

Video: FBI confirms bomb planted at Spokane parade (on this page)

"Hundreds of people, including children, gathered to celebrate and recommit their lives to the cause of human rights," Richard said.

Aryan Nations
Spokane has 200,000 residents and is about 100 miles south of the Canadian border.

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, and agents didn't know if the two incidents were related, Harrill said.

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 the Aryan Nations, whose leader Richard Butler gathered racists and anti-Semites at his compound for more than two decades. Butler went bankrupt, lost the compound in a civil lawsuit in 2000 and died in 2004.

In 1996, white supremacists placed a pipe bomb outside City Hall in Spokane. The bomb exploded, blowing out a window and sending nails and screws across the street.

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.

© 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Video: FBI, police seek help in MLK parade bomb plot

  1. Closed captioning of: FBI, police seek help in MLK parade bomb plot

    >>> co." a call for help in washington state this morning. police want to know if anyone saw anything during a martin luther king parade on monday. a potentially lethal bomb was found in a backpack and now the search is on for who planted it. pete williams joins us from washington. pete, do investigators have any significant leads?

    >> that's what they say, chris. one of the avenues they're exploring is whether this was the work of white supremacist groups that are in that area of the country. white supremacist or racist groups. but you said you talked about seeing something and saying something, that's how this device was discovered before the start of monday's unity parade in spokane. it was seen by some city workers and safely defused and that in itself is a lead for investigators because the bomb wasn't destroyed, so they're analyzing it now. they're also putting out pictures of what the bomb was in. it was in a black swiss army brand backpack and wrapped in two t-shirts and they're hoping the shirts might jog somebody's memory. they have pictures of the shirts. one is for an area relay race this year and the other says treasure island spring of 2009 and they're hoping that that may jog somebody's memory. they may know somebody who had shirts like that or at that event and had a backpack like that. to help sweeten someone's memory the fbi is offering a reward of $20,000 for information to help crack this case. they're also asking for anybody that was in that area. the corner where this device was placed at the time it was put there, which they say was between 8:00 in the morning and 9:30 on monday morning, chris.

    >> horrible, horrible stuff. thank you so much, pete. i know you'll keep us updated on the investigation.

    >> you bet.

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