Out-of-control dump truck barrels down Seattle street before crashing into Subway restaurant, injuring several people

Authorities in Seattle are pointing to a "catastrophic mechanical failure" that they say caused an out-of-control dump truck to speed through a downtown square, injuring several people, before slamming into a Subway restaurant.

"At this point, we're operating under the assumption that there was some kind of mechanical failure in the truck. But again, it's still very early in the investigation so this could change, especially as our detectives start taking a look at the truck," detective Patrick Michaud told ABC News Seattle affiliate KOMO-TV on Monday.

Just after 9:30 a.m. on Monday, police and firefighters responded to calls about a dump truck colliding with people and vehicles as it sped down James Street in the city's Pioneer Square.

Vonda Sargent, a Seattle lawyer, told KOMO-TV that she was walking across the street to a deposition when she heard a truck that sounded completely out of control.

"I have my earbuds on and I look up and I see a truck coming down striking a bunch of vehicles, vehicles striking one another," she said. "I see this truck barreling right towards me so I run across the street. ... Just as the dump truck slams into the Subway, completely into the Subway. ... People were fleeing. ... The dump truck did not slow down. ... The only thing that stopped it was the wall in the Subway. You could see the dump truck hit the wall and bounce back."

Sargent said that the truck hit the cars so hard that the impact left them spinning. She said people were running for help.

When authorities arrived, they found that one person and three cars — one that had been parked and two that were occupied — had been hit by the dump truck before it went into the Subway restaurant.

"The [truck] driver reportedly attempted to steer the truck through traffic. ... One employee was inside of the restaurant at the time of the crash and was taken to Harborview Medical Center with minor injuries," Seattle police said in a news release.

The pedestrian who was hit — a 33-year-old man — was taken to the hospital with serious injuries but was said to be in stable condition. The driver of one of the moving vehicles — a 25-year-old woman — was also transported to the hospital in serious condition, the fire department said.

Two others — a 51-year-old man and 25-year-old man — were in stable condition at the hospital but the fire department said it was not clear where the two were at the time of the crash.

The dump-truck driver was not hurt, the fire department said.

Seattle police told KOMO-TV that the square was shut down for the afternoon as the investigation continued.

"Crews worked to secure the building to ensure it was structurally sound prior to removing the dump truck," the fire department said.

ABC News' Jennifer Watts contributed to the reporting in this story.