PHILADELPHIA (CBS) -- Two people are dead after a Mercedes traveling at high speed on the Roosevelt Boulevard collided with a school bus in Northeast Philadelphia.

Upon impact, the school bus burst into flames as the bus was stopped at a red light.

The school bus belonged to the Huntingdon Valley Christian Academy.

Just prior to the accident, the 40-year-old driver dropped off 28 kids after attending an event. No children were on the bus at the time of the accident, the bus driver was not injured.

Two off-duty officers were in the area and witnessed the crash.

The exact cause of the collision remains under investigation.