Five people, including two children, are confirmed to have died after a semi-truck carrying a toxic substance overturned in Illinois, releasing a cloud of ammonia that prompted the evacuation of area residents, authorities said on Saturday.

Kim Rhodes, the Effingham county coroner, confirmed to the station WCIA that the fatalities came as a result of Friday night’s accident in Teutopolis, Illinois.

Rhodes said the children and one adult are from one family; the other two fatalities are from Missouri and a semi-truck driver from Ohio who was passing the crash scene.

Another five people were airlifted from the area for treatment at central Illinois hospitals. One of those airlifted victims is the driver of the truck carrying the ammonia who is being treated at a Springfield, Illinois, hospital.

Earlier on Saturday, the Illinois state police said the evacuation orders in Teutopolis will remain in place after the town’s fire chief said the crashed tanker was still half full of ammonia and remained at the crash site.

The Illinois environmental protection agency (IEPA) said the tanker was carrying approximately 7,500 gallons of anhydrous ammonia and early estimates indicate approximately 4,000 gallons had been released.

The state agency added that IEPA approximately 500 individuals have been evacuated from the area due to the leak, reported Wand news.

The National Transport Safety Board (NTSB) said it was sending a 15-person team including Thomas Chapman, one of the safety agency’s five board members, to Teutopolis.

Firefighters, police and other emergency responders converged on the scene late Friday to try to contain the cloud of ammonia leaking from the overturned tanker.

“We have a lot of brave firemen, EMT, hazmat specialists, police officers that are working on this scene as we speak,” the Effingham county sheriff, Paul Kuhns, said at a Saturday press conference.

Authorities did not say exactly how many people were killed or injured.

“I am sorry to say that we do have multiple fatalities,” Kuhns said. “And I’m sorry to say I don’t have that exact number for you.”

A spokesperson for the Illinois state police, Melaney Arnold, said authorities initially thought there were five fatalities, but that turned out to be a premature determination amid the early confusion.

The accident caused “a large plume, cloud of anhydrous ammonia on the roadway that caused terribly dangerous air conditions in the north-east area of Teutopolis,” Kuhns said. “Because of these conditions, the emergency responders had to wait. They had to mitigate the conditions before they could really get to work on it, and it was a fairly large area.”

The accident, which involved “multiple” vehicles including the tanker, happened about a half-mile east of Teutopolis on US Highway 40 on Friday at about 9.25pm, Illinois state police said in an emailed statement.

Tim McMahon, the chief of the Teutopolis fire protection district, said the tanker began leaking after rolling over in a ditch. He said authorities were still preventing cars from driving in that area.

Kuhns said he did not have information on whether the deaths were the result of the crash itself or the chemical leak. But he said that “the accident scene was large”.

He apologized for any inconvenience resulting from the investigation and the evacuation.

“I understand how frustrating that would be to not be allowed to travel or to go back home, but we really need to focus our resources on the spill and on the situation, so if people can have as much patience, that’s what I would ask for,” Kuhns said.

There was an earlier accident on Interstate 70 on Friday, and authorities had detoured traffic onto US Highway 40, including the tanker.

Anhydrous ammonia is toxic and can be a health hazard if safe handling procedures are not followed. Effects of inhalation of anhydrous ammonia range from lung irritation to severe respiratory injuries, with possible death at higher concentrations. Anhydrous ammonia is corrosive and can burn the skin and eyes.

Teutopolis is about 92 miles (148km) south-east of Springfield, the capital of Illinois.