BEACH PARK, Ill — Plumes of a toxic gas that leaked in a Northern Chicago suburb Thursday morning sent at least 37 people to hospitals and prompted an order for residents to stay inside their homes with windows shut tight, officials said.

Lake Forest Fire Chief Mike Gallo said a tractor was towing two separate two-ton containers of anhydrous ammonia when the leak occurred around 4:30 am.

Initial reports suggested the vehicle was involved in a crash, but the sheriff's office later said that was not the case. The cause of the leak has not been released.

The leak created a toxic cloud that lingered for several hours over Beach Park, about 40 miles (65 kilometers) north of downtown Chicago.

Eleven firefighters were among the 37 people hospitalized, and one of the injured firefighters was among seven people in critical but stable condition, according to the Lake County Sheriff's Office.

Three law enforcement officers were in good condition and several others were serious but stable, sheriff's spokesman Christopher Covelli said. Authorities said most of those injured suffered breathing problems.

"This is a very dangerous chemical that can cause unconscious and worst-case scenario death," Covelli said.

According to the Centers for Disease Control and Prevention, anhydrous ammonia is a colorless gas that can cause breathing difficulties, burns, blisters and is fatal if breathed in high concentrations. Farmers use it to add nitrogen to soil.

Plumes of a toxic gas that leaked in a northern Chicago suburb Thursday morning sent dozens of people to hospitals and prompted an order for residents to stay locked inside their homes with windows shut tight, officials said. 

The first two officers who responded to the leak had to retreat because they were overcome by the ammonia, Covelli said.

"It was difficult to get to that scene initially with those chemicals in the air without proper protection and gear to wear over the face," he said.

Authorities said about two dozen law enforcement agencies responded to the leak. The leak was contained within a few hours and the leaking tank was empty, authorities said, adding that they were waiting for the chemical plume to dissipate.

Residents within a 1-mile radius of the leak were initially told to close their windows and remain indoors, but that order was lifted at 10 am, Covelli said. Later Thursday morning, officers went door-to-door in the area to check if anyone was in medical distress, Covelli said.

AP