An FBI investigation is underway to find who left a backpack bomb along a planned route for a Martin Luther King Jr. Day parade in Spokane, Wash.

The bomb was defused without explosion on Jan. 17, according to the Associated Press. An anonymous government official told the AP that the device was the most potentially destructive bomb he’d ever seen in America.

“They haven’t seen anything like this in this country,” the official said. “This was the worst device, and most intentional device, I’ve ever seen.”

The bomb was found by three city employees about an hour before the downtown parade was to start, Frank Harrill, special agent in charge of the FBI Spokane office, told the Seattle Post-Intelligencer. The employees looked inside, saw wires and immediately alerted law enforcement, and the parade was rerouted, according to the paper.

“I just found it so telling that here we are celebrating the life of a person who espoused non-violence in all its forms and yet we’re going to have a bomb,” Al Odenthal, former deputy chief with the Spokane Police Department, told Spokane ABC affiliate KXLY-TV.

Harrill, agreed with Odenthal, but told the AP that the failed bomber’s motives were not necessarily clear. The region has been home to the white supremacist group Aryan Nations.

“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.

KXLY-TV reported that FBI investigators are tracing two tee shirts left in the backpack with the device. The tee shirts were made by the American Cancer Society for a “Relay for Life” race at Colville High School just north of Spokane.

About 40 copies of the shirts were given to students who participated in a March 2009 performance of “Treasure Island” in the Chewelah School District, but officials say none of the students are currently suspects.