Five people died after a crash involving multiple vehicles, including a tractor-trailer carrying anhydrous ammonia that overturned in Illinois on Friday night, leading to a leak of the toxic gas and prompting residents within a one-mile radius of the crash site to evacuate, officials said.

The vehicle rolled over on U.S. Highway 40, about half a mile east of Teutopolis, Ill., which is about 90 miles southeast of Springfield, around 9:25 p.m. on Friday, releasing a large plume of anhydrous ammonia and causing dangerous air conditions, officials said Saturday morning at a news conference.

Kim Rhodes, the Effingham County coroner, said five people had died as a result of the crash, adding that it was possible additional fatalities could be reported later. It was not immediately clear how the victims died.

Three were from the area, including a child and an adult who were from the same family, Ms. Rhodes said. Two other victims were from Ohio and Missouri, she said.

Chief Tim McMahon of the Teutopolis Fire Protection District said that five people with injuries were airlifted to hospitals and that another victim was taken to a hospital in a vehicle. Their conditions were not immediately known.

The Illinois Environmental Protection Agency said on Saturday afternoon that the vehicle was carrying 7,500 gallons of anhydrous ammonia and that about 4,000 gallons had been released.

Chief McMahon said at the news conference that a hazmat team had patched the ruptured part of the tanker, which slowed but did not stop the leak. As of Saturday afternoon, the leak was still partly patched, and crews were working to identify the safest way to empty the tanker, the Illinois E.P.A. said.

About 500 people were evacuated from the area, the agency said, but residents were allowed to return on Saturday evening. About 1,500 people live in the village of Teutopolis. Officials advised residents to air out their homes out by opening all windows.

“If you feel there is a strong odor or start to feel sick, call 911 immediately to have the residence checked by fire personnel,” Effingham County emergency officials said.

Because of the dangerous plume of gas, emergency crews had to wait before responding to the crash, Sheriff Paul Kuhns said.

“They had to mitigate the conditions before they could really get to work on it, and it was a fairly large area,” he said.

The National Transportation Safety Board was sending a team to help with the investigation.

Anhydrous ammonia — often used in manufacturing, refrigeration and agriculture — is a toxic gas that can be corrosive if people have contact with it, according to the Centers for Disease Control and Prevention.

“It’s terrible,” Sheriff Kuhns said. “It’s bad stuff if you are involved in it — breathe it, especially — because it gets in your airways, in your lungs.”

Environmental Protection Agency workers were expected to arrive later on Saturday to monitor air quality.

Chief McMahon said that shifting wind directions had further complicated the response to the crash. Crews were set up in multiple locations to respond to the gas leak based on the wind changes, he said.

Illinois State Senators Steve McClure, Jason Plummer and Chapin Rose said in a statement that they were monitoring the situation and expressed condolences to the victims’ families, adding, “Please stay clear of the area and allow first responders to work.”