ATTEMPTED DOMESTIC TERRORISM SUSPECTED IN SPOKANE…. This could have been absolutely devastating.

A “potentially deadly” explosive device that could have caused severe casualties was found along the intended route of a Martin Luther King Day march in Spokane, Wash., half an hour before the event was to begin, the FBI said Tuesday.

The annual Unity March was rerouted after city workers noticed a black Swiss Army backpack apparently abandoned on a bench about 9:25 a.m. Monday, said Frank Harrill, the supervisory senior resident agent in the FBI’s Seattle division.

The device inside “clearly would have had the potential to inflict multiple casualties, injury and death, to humans,” Harrill said in an interview Tuesday.

Some local workers noticed the unattended bag on Monday, and notified police after seeing wires coming out of the backpack. Organizers rerouted the parade, while the bomb squad used a robot to dismantle the apparent bomb. According to several accounts, this was a fairly sophisticated device, with a remote detonator, placed in such a way as to maximize the damage to those marching in the parade.

Given the circumstances, the FBI’s Harrill said officials are “treating this as an act of domestic terrorism,” and consider the connection to the King Day march “inescapable.”

“At that point, it falls directly in the realm and sphere of domestic terrorism,” Harrill told the AP. “Clearly, there was some political or social agenda here.”

It’s also worth emphasizing the area’s history of white supremacist activity: “Until 2001, the Aryan Nations was headquartered in nearby Hayden Lake, Idaho. As recently as April 2009, the Spokesman-Review newspaper reported that residents of a Coeur d’Alene, Idaho, subdivision had found Aryan Nations recruitment letters on their lawns.” Last March, some kind of explosive device was also found outside the local federal courthouse.

Rachel Maddow’s report on this last night is worth watching:

Visit for breaking news, world news, and news about the economy

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.