BEACH PARK, Ill. (AP)–The National Transportation Safety Board is sending a team to Illinois 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 on Friday, when it will begin work.

Authorities say a tractor was towing two separate two-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.

The leak created a toxic cloud that lingered for several hours over Beach Park, about 40 miles (65 kilometers) north of downtown Chicago. Authorities ordered residents within a 1-miles radius to stay inside and close their windows. Area schools were closed for the day.