Federal agents are investigating race as a possible motive behind an abandoned backpack containing a functional bomb after it was left along the downtown route of a Martin Luther King Jr. Day parade in Spokane, Wash.
Investigators would not disclose what kind of explosive it was, except to say that it was "potentially deadly" and could have caused "multiple casualties" had the device detonated.
While the FBI hasn't provided direct evidence that the explosive device was connected to the march, an agency spokesman said the backpack's proximity to the route was "not coincidental."
"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," said Frank Harrill, special agent in charge of the Spokane FBI office.
The suspicious backpack was spotted by three city employees at an intersection in downtown Spokane about an hour before the parade was to start Monday, Harrill said. They saw wires and immediately alerted law enforcement, who disabled it without incident, he said.
The FBI released a photo of the Swiss Army backpack as it sought information from the public. Also released were pictures of two 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.
The FBI and local officials have praised as heroes the city workers who spotted the backpack and quickly called police. Police were also hailed for immediately deciding to reroute the parade. The several hundred marchers, including many children, were not told why the route was changed.
Harrill said the FBI has received some leads since offering a $20,000 reward for information on Tuesday. But the agency can't discuss the leads publicly, he said.
There were no notes in the backpack, which has been shipped to an FBI lab in Quantico, Va., Harrill said.
Investigators are also seeking anyone who took photographs or video in the area between 8 a.m. and 11 a.m. on Monday.
The discovery before the 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.
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 nonviolence, we were faced with a significant violent threat," Verner said. "This is unacceptable in our community or any community."
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 the 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.
Harrill decried the planting of the bomb as an act of domestic terrorism that was clearly designed to advance a political or social agenda.
"The potential for injury and death were clearly present," he said of the bomb.
The FBI received no warnings in advance and did not have a suspect, Harrill said. No one has claimed responsibility for planting the bomb.
NPR's Martin Kaste reported from Seattle for this story, which contains material from The Associated Press