The company that owns the truck now says the brakes failed, sending the driver and his co-worker on a downhill collision course.
Dozens watched as the truck slammed into four vehicles.
Witnesses reported one car exploded and video from the scene at 216 James Street showed flames on that car shooting a dozen feet high, creating a dark smoke column that could be seen for several blocks.
"Most of the people tried to get closer to see if they could do anything, but after they saw that the fire was huge - couldn't do anything," bystander Brian Oliveira said.
Fire officials said there were only minor injuries, but they were taking the driver of the truck to Harborview Medical Center as a precaution.
A total of five vehicles were damaged, with three having caught fire.