O'Fallon, IL (KSDK) - Illinois State Police Accident Reconstruction investigators continue to evaluate the Thursday afternoon crash that left a disabled passenger on board dead.

O'Fallon Police Sgt. Eric Andrews tells NewsChannel 5 the incident occurred around 4:20 p.m. Thursday in the 800 block of Matthew Drive.

The driver of a SAVE transport bus experienced a medical emergency and left the roadway before slamming into a tree.  SAVE stands for St. Clair Associated Vocational Enterprises, and provides general transportation to developmentally disabled adults to and from their work places.

Along with the bus driver, two clients and a bus monitor were on board at the time of the crash.

Brian McCarty, a 54-year-old client of the service, was killed in the collision.  The female bus monitor was slightly injured and was treated and released at a local hospital.  The second client was not hurt and was picked up by family members.

The driver of the bus was taken by air ambulance to a St. Louis hospital.  His condition is not known at this time.

KSDK