Rush hours will move markedly slower for Chicago Transit Authority bus riders starting this evening.

Agency officials say they found a crack in one of the fleet’s 60-foot, accordion-like buses, and that they pulled all similar models off the streets. Ninety operated near the lakefront during rush hours, officials say.

The biggest delays will be from 7 to 8 on weekday mornings and from 4:30 to 5:30 on weekday afternoons, officials say. They advise riders to allow extra time for their commutes and consider traveling on rush-hour trains added to the Red, Blue, Brown, Green and Pink lines.

"We deeply regret the inconvenience," Acting CTA President Dorval Carter said in a statement. “The safety of bus riders has to be our priority."

The agency said it was unclear when the buses would be returned to service.

The cracked bus is one of 226 the CTA purchased from North American Bus Industries since 2003, officials say, and it was about five years old with 152,000 miles. The buses come with 12-year, 500,000-mile warranties.

North American is not the sole manufacturer of CTA’s articulated buses. New Flyer of America Inc. has been supplying the agency with about three of the long buses per week, so far about 94.

Those buses are still running.