King County Sheriff’s Office, this dud’s for you.

A carefully crafted dummy pipe bomb that was inadvertently left behind after a Sheriff’s Office training exercise last week sparked an all-too-real bomb scare Monday morning when a passenger found it aboard a King County Metro bus.

The bus and nearby businesses in Kent’s East Hill neighborhood were evacuated and streets were closed for several hours while the sheriff’s bomb squad and a SWAT team responded.

A few hours after the report, the Sheriff’s Office realized that the “bomb” may have been one of theirs.

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“It’s embarrassing. We know it’s inconvenienced a lot of people, and we don’t take it lightly,” said sheriff’s spokeswoman Sgt. DB Gates. “We’re more than willing to throw ourselves on our sword for this one.”

The dummy pipe bomb was part of a training exercise Thursday designed to test the response of deputies to a terrorist bombing, Gates said. The training scenario called for an attack on multiple Metro coaches, resulting in numerous victims, according to the Sheriff’s Office.

Deputies were supposed to have double- and triple-checked the buses to make sure all the training devices were removed after the drill, said Gates, but that apparently failed to happen.

According to Gates, a passenger got on the Route 161 bus shortly before 7:45 a.m. Monday and found a zipped backpack under a seat. The passenger looked inside and saw the realistic-looking dummy bomb, Gates said.

The passenger did “exactly what he was supposed to do” and reported the device to the bus driver, who also called police and evacuated the bus at 104th Avenue Southeast just north of Southeast 240th Street, Gates said.

It wasn’t until about 11:15 a.m. that the Sheriff’s Office realized that the device may have been left over from last week’s drill, she said.

“But we couldn’t just say it’s a fake device,” she said. Gates said protocols still had to be observed and the device had to be treated as if it were a potential threat.

Gates said the bomb squad used a robot and a “water dispersement device” that broke the phony pipe bomb into pieces.

“All we can do now is apologize for how that went down and change our protocol,” she said.

A spokesman for King County Metro said it appears the bus had not been in use since last week’s drill, which explains why the backpack wasn’t spotted until Monday morning.

Christine Clarridge can be reached at cclarridge@seattletimes.com or 206-464-8983.