CHICAGO (CBS) – There is no official word Sunday morning on what caused a CTA bus with 70 passengers aboard to run off the road and into several trees on the the South Side – sending 37 people to hospitals, including four in serious to critical condition.

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

“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.’’

CTA spokeswoman Noelle Gaffney said Sunday morning there is no official cause of the crash, even a preliminary one.

“We don’t have that yet. We will look at what the operator said, the black box which tells you what speed it was going, and how the systems on the bus were working and the mechanicals,’’ Gaffney said.

Gaffney said they are also looking at video to see what was happening on the 60-feet long bus prior to the crash. “That gives us a sense of the chain of events,’’ Gaffney said.

CTA officials will also be taking statements from people on the bus. The process of determining the cause will not happen immediately.

“There are a lot of steps we have to take,’’ she said.

The wreck shut down three lanes of southbound Lake Shore Drive. Gaffney said while it was a full bus, it was not “packed” as it can hold up to 110 passengers.

Gaffney did not have a condition update Sunday morning, but initially after the crash, four people were taken in serious-to-critical condition to Northwestern and Stroger Hospitals, while 10 were transported in fair-to-serious condition to area hospitals, fire department officials said. Another 23 were transported to hospitals with minor injuries.

All but one of the people taken to Stroger Hospital had been treated and released by late Saturday, hospital spokesman Lucio Guerrero said. The one remaining person hospitalized is now in “stable” condition with an arm injury.

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.

Gaffney could not confirm passenger reports on the scene which speculated that the weight of the large amount of people aboard actually helped by keeping the bus from tipping over.

“Well, the way the bus stopped, it was in some trees. So it was still level — it wasn’t on an angle or anything,’’ she said.

The crash caused heavy damage to the right side of the front of the bus. The bus was in the process of being towed to the CTA’s 77th Street garage for inspection late Saturday.

The female driver will undergo blood and alcohol testing, Rodriguez said. He could not say how long she had been a driver or whether she had been involved in an previous collisions.

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