Police cited the driver of a charter bus that rear-ended a semitrailer, sending 17 people to hospitals, authorities said Wednesday.

The bus driver was ticketed for following too closely behind another vehicle, but the cause of the crash late Tuesday on the Chicago Skyway remained under investigation, Chicago Police spokesman John Henry said.

"That is not necessarily the cause of the crash," he said. "That might have been a contributing factor, hence the ticket."

The driver's name was not released.

About three dozen people were on the private charter bus when it hit the semitrailer, Chicago Fire Department spokesman Kevin MacGregor said.

At least 13 patients had been released from local hospitals by Wednesday morning, authorities said.

The drivers of the bus and the semitrailer both refused treatment, MacGregor said.

Henry said the tour bus was operated by Chicago Entertainment Tour, but he did not know where the bus was headed or where it originated. A message left with the company was not immediately returned Wednesday.