Five people were injured Monday when a dump truck crashed into a Subway restaurant in Seattle's Pioneer Square.

The truck allegedly had "a catastrophic mechanical failure" and began rolling forward and struck a pedestrian at about 9:30 a.m., according to the Seattle Police Department. The driver then reportedly attempted to steer the truck through traffic but struck three cars before slamming into the Subway, police said.

At least four people, including the pedestrian, were seriously injured, according to the Seattle Fire Department. One Subway employee inside the restaurant at the time was taken to a hospital.

The driver was not injured, police said.

The fire department said that it was evacuating the building so that crews could begin work on extricating the dump truck from the building.

A witness told NBC affiliate King 5 News that the crash trapped a woman in her van and witnesses rushed to help her while others called police.

"It was crazy. I've never seen anything like that in my life," Kyle Lewis, who was working nearby, told King 5 News.

"All of the sudden, I saw a dump truck coming about 40 miles an hour, with lost brakes, took out these two cars and hit someone up the street that was crossing the street," Lewis added. "And then, he came flying right through here and went straight into the Subway. It happened within, like, 5 seconds."