Updated at 9:53 a.m.

The National Transportation Safety Board is sending a team to a suburb north of Chicago to investigate a chemical release from two containers that caused a toxic plume that sickened dozens of people.

The NTSB announced the team of four will be on the scene in Beach Park, near Zion, on Friday.

Authorities say a tractor was towing two separate 2-ton containers of anhydrous ammonia when the leak occurred around 4:30 a.m. Thursday. Initial reports suggested the vehicle was involved in a crash, but the sheriff’s office later said that was not the case. Further details on the cause of the accident have not been released.

The leak created a toxic cloud that lingered for several hours over Beach Park. The Centers for Disease Control says anhydrous ammonia is a colorless gas with pungent, suffocating fumes, used as an agricultural fertilizer and industrial refrigerant.

Officials ordered residents within a 1-mile radius to stay inside and close their windows. Area schools were closed for the day. Authorities say 11 firefighters and three police officers are among 37 people hospitalized due to an ammonia leak in a northern Chicago suburb. The Lake County Sheriff’s Office says seven of those hospitalized are in critical but stable condition, including one firefighter.

Sheriff’s spokesman Christopher Covelli says the three officers are in good condition. Covelli says the first two officers who responded to the leak had to retreat because they were overcome by fumes.

Lake County Sheriff John Idleburg released a statement Thursday night commending “all of our Lake County Sheriff’s Deputies, police officers from nearly twenty area police departments, and all of the firefighters who risked their lives today to help others. You are an amazing group of men and women.”

He said Lake County detectives and deputies are investigating how the leak occurred.