This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

CHICAGO — Four people were hurt when a CTA bus traveling north on Lake Shore Drive swerved across the south bound lanes, crossed traffic going in the opposite direction and crashed into a tree Monday afternoon.

Police say the articulated CTA bus was carrying 34 passengers around 2 p.m. Monday when the driver is believed to have experienced a medical emergency, causing them to suddenly swerve across the south bound lanes near the Lawrence Avenue exit. The bus crossed a divider before hitting a tree in a nearby park.

A person believed to be the driver was hurt and taken to a hospital in critical condition, while three others were hurt and taken to hospitals in fair condition, according to police and fire officials. Four other people were assessed at the scene and refused treatment.

Watch: Video from Skycam9 shows the scene of a crash involving a CTA bus