Seven people are in critical condition and several more were hospitalized after a gas leak in a northern Chicago suburb early Thursday morning caused a toxic cloud to linger for several hours, officials said.

A tractor towing two, two-ton containers of anhydrous ammonia about 4:30 a.m. sustained a leak in one of the tanks, creating a chemical cloud that lingered over Beach Park, a suburb 40 miles north of Chicago, Lake Forest Fire Chief Mike Gallo said.

Police say about two dozen law enforcement agencies responded to the leak, causing many of them to be hospitalized.

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.

Thirty-seven people were hospitalized after the leak, many suffering breathing problems. Eleven firefighters were among those sent to the hospital and one was listed 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 in serious but stable condition.

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Authorities have not released what caused the leak but said it was contained within a few hours.

During the leak, residents within a one-mile radius were urged to stay indoors and shut their windows while schools were closed for the day.

The Associated Press contributed to this report.