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

The discovery before Monday's parade for the slain civil rights leader raised the possibility of a racial motive in a region that has been home to the white supremacist Aryan Nations.

"The confluence of the holiday, the march and the device is inescapable," said Frank Harrill, special agent in charge of the Spokane FBI office. "But we are not at the point where we can draw any particular motive."

He called the planting of the bomb an act of domestic terrorism that was clearly designed to advance a political or social agenda.

The suspicious backpack was spotted by three city employees about an hour before the parade was to start, Harrill said. They looked inside, saw wires and immediately alerted law enforcement.

The parade route was changed to avoid the device. A bomb disposal unit disabled it without incident, he said.

Spokane Mayor Mary Verner said the attempted bombing was unacceptable.

"I was struck that on a day when we celebrate Dr. King, a champion of non-violence, we were faced with a significant violent threat," Verner said. "This is unacceptable in our community, or any community."

Harrill declined to release details of the device, other than to call it a functional bomb that could have caused multiple casualties.

"The potential for injury and death were clearly present," Harrill said.

The FBI received no warnings in advance and did not have a suspect, Harrill said. No one has claimed credit for planting the bomb.

The agency decided to appeal to the public for information and offered the $20,000 reward for information leading to an arrest and conviction.

"Subject should be considered armed and dangerous," the agency said in its announcement.

The agency released photos of the backpack and two shirts found inside.

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 1996, white supremacists placed a pipe bomb outside City Hall. The bomb exploded, blowing out a window and sending nails and screws across the street into Riverfront Park. There were no injuries. Two men arrested for the blast said it was part of a plot to spark a revolution and set up a whites-only nation.

In December, a man in nearby 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.