The fiery multi-car crash that killed four people in Thornton on Saturday was caused by a rear-end collision at a red light, police said. A Ford SUV carrying four people was traveling northbound on Washington Street when the driver stopped at a red light at Thornton Parkway, according to a Tuesday news release from the Thornton Police Department. Police said the driver of the second car, a Subaru sedan, was speeding down northbound Washington Street when it crashed into the back of the stopped SUV around 7:30 p.m.