Authorities say dozens of people have been injured in Chicago after a crowded commuter bus traveled off a major thoroughfare and crashed into trees.

Fire Department spokesman Larry Langford says around 70 people were on the Chicago Transit Authority bus when it veered off Lake Shore Drive around 6 p.m. Saturday.

The impact caved in a front side of the bus.

Langford says one person was taken to a hospital in critical condition and three in serious condition. About a dozen others were in fair to serious condition.

Langford says more than 20 others were treated at the scene mainly for bumps and bruises.

The spokesman says it's not known what caused the accident. He says the CTA is investigating.