Exposure to anhydrous ammonia apparently caused the deaths of five people, including two children, when a tanker truck loaded with the hazardous material overturned on a highway Friday night in a rural Illinois community, according to preliminary findings from the local coroner's office.

At least seven other people from six different states were also treated at hospitals after being overcome by what authorities described as a "large plume cloud" that was released when the tanker truck spilled its load on a highway east of Teutopolis, Illinois, Effingham County Coroner Kim Rhodes said in a statement Sunday evening.

Autopsies are scheduled to be performed Monday morning on the victims to confirm the preliminary findings, Rhodes said.

"Preliminary investigation indicates five individuals died from exposure to anhydrous ammonia at the crash site," according to Rhodes' statement.

Three of the people killed were from the same family.

Those killed were identified by the coroner's office as 34-year-old Kenneth Bryan of Teutopolis and his two children, 7-year-old Rosie Bryan and 10-year-old Walker Bryan, both of Beecher City, Illinois.

Danny J. Smith, 67, of New Haven, Missouri, and Vasile Crivovan, 31, of Twinsburg, Ohio, also apparently succumbed to exposure to the anhydrous ammonia, according to the coroner's preliminary investigation.

The deadly highway wreck unfolded around 8:40 p.m. local time Friday when the semi-truck rolled over on U.S. Route 40 and spilled about 4,000 gallons of anhydrous ammonia on the roadway, causing "terribly dangerous air conditions," Effingham County Sheriff Paul Kuhns told reporters on Saturday.

Anhydrous ammonia is a clear, colorless gas that is toxic. Effects of inhalation range from nausea to respiratory tract irritation, depending on the length of exposure, according to the Centers for Disease Control and Prevention. The chemical is primarily used in farming as a nitrogen fertilizer.

Rhodes said the victims were exposed to the ammonia "due to traveling through the scene of the crash site."

Seven people, including four teenagers, were treated at area hospitals for exposure to the anhydrous ammonia, including two who were admitted to hospitals, according to the coroner's statement.

About 500 residents living within roughly 2 square miles of the crash site were initially evacuated, authorities said. They were allowed to return to their homes on Saturday after the danger from the ammonia spill dissipated, Teutopolis Assistant Fire Chief Joe Holomy said in a statement.

The cause of the crash remains under investigation.

The National Transportation Safety Board, in coordination with the Illinois State Police and the Effingham County Sheriff's Department, sent a 15-person team to conduct a safety investigation into the rollover crash, the agency said Saturday.

Representatives from the U.S. Environmental Protection Agency also responded to the scene.

Teutopolis is a small village in Effingham County, located about 92 miles southeast of Springfield, the capital of Illinois.