SEATTLE (Reuters) - The FBI is asking the public for help in the investigation of an unexploded backpack bomb left along the parade route of a Martin Luther King Jr. holiday celebration in Spokane, Washington, the agency said on Tuesday.

The unattended Swiss Army-brand backpack, with wires visible, was discovered on a downtown bench by three city workers who notified police of the device about 30 minutes before the parade was scheduled to begin.

The parade, attended by about 1,500 people, was quickly rerouted while city’s bomb disposal unit was summoned and safely “neutralized the device,” the FBI said.

FBI agent Frank Harrill described the “improvised explosive device” as having a “very lethal design” capable of inflicting “multiple casualties.”

The device in the backpack was largely concealed by two T-shirts packed inside. The exact design and suspected strength of the bomb were not revealed.

The FBI offered a $20,000 reward for information leading to the arrest and conviction of any individuals responsible for the bomb. Investigators also asked anyone who took photographs or video footage in the area where the pack was found during a three-hour period before the parade to contact the FBI.