Transcript for Toxic cloud in Illinois injures several people
sit down on Tuesday on GM "Good morning America." Next tonight, a toxic cloud that forced people to shelter inside. A tractor was towing containers of ammonia when one of the tankers sprang a leak. Firefighters hosing it down fur hours. Officers were checking on people, going door to door. ABC's Alex Perez is there. Reporter: An investigation underway tonight in the suburbs north of Chicago to determine what caused this tank to leak a hazardous chemical right near a residential area. The incident, starting around 4:30 this morning, prompting a massive response from authorities. There is some type of ammonia smell here. Reporter: A sergeant and deputy arriving on-scene overcome, forced to retreat, quickly realizing the air wasn't safe. I do have three patients on the ground. Reporter: 37 people hospitalized. Seven in critical but stable condition, including one firefighter from the toxic cloud created by anhydrous ammonia used in farm fertilizer. The trailers and the containers holding the chemical leaked, and this leak caused plumes of toxic gas to be released into the air. Reporter: Authorities at one point calling for residents to shelter in place. William Haske opened his front door and was overcome by the haze and smell. We couldn't see beyond the first step here, it was that thick. We couldn't see three feet, it was heavy. It was like it was a fog, but it was real strong. Reporter: Schools nearby closed. Crews blocking off a one-mile perimeter as they monitored the area. We went around and made sure all our windows were shut and all our storm windows were tight. Reporter: Six hours later, authorities finally giving the all clear. And Tom, the NTSB wilbe sending a team here to investigate what exactly triggered all this, and if that chemical was being legally you can see, finally, that intersection here is back open. Tom? Alex Perez tonight. Alex, thank you.
This transcript has been automatically generated and may not be 100% accurate.