CHICAGO (STMW) Thirty-seven people were hospitalized -- including four in serious-to-critical condition -- after a packed CTA bus ran off South Lake Shore Drive and crashed into several trees.

The #6 Jackson Park Express bus, an articulated bus, was headed southbound when it crashed into a grassy area at about 6 p.m. near the 2800 block of South Lake Shore Drive, Chicago Fire Department spokesman Larry Langford said.

Officials were still investigating the cause of the crash.

"The driver said the bus veered to the right,'' Rich Rodriguez, president of the CTA, said at the scene. "It appears that the trees were what stopped the bus from going any further.''

Seventy people were aboard the bus at the time of the crash, which shut down three lanes of southbound Lake Shore Drive.

Four people were taken in serious-to-critical condition to Northwestern Memorial Hospital and John H. Stroger, Jr. Hospital of Cook County. Ten were transported in fair-to-serious condition to area hospitals, fire department officials said. Another 23 were transported to hospitals with minor injuries.

The driver was taken to Rush University Medical Center in good condition.

Thirty-three riders refused to be transported and were either not injured or were treated at the scene and released, officials said. Another #6 bus picked up passengers and continued with its normal route,  CTA officials said.

There were no other vehicles involved in the crash.

The crash caused heavy damage to the right side of the front of the bus.

Rodriguez said officials would check the "black box'' on the bus and also check its mechanics to see if there was a mechanical error.

The female driver will undergo blood and alcohol testing Rodriguez said.

Saturday evening, the bus was in the process of being towed to the CTA's 77th Street garage for inspection.