Five people killed after a multi-vehicle crash in Illinois died from exposure to a toxic chemical that spilled from an overturned semi-truck, the Effingham County Coroner's Office said Tuesday.

A preliminary investigation shows the fatalities were the result of the "toxic chemical anhydrous ammonia exposure," the coroner office told CNN via email. "No official cause of death will be released until the final autopsy report is completed."

The victims were identified as Kenneth Bryan, 34, and his two children -- Rosie, 7, and Walker, 10 -- along with Danny J. Smith, 67, and Vasile Cricovan, 31, according to the coroner's office.

The accident appears to have started when another vehicle tried to pass the semi-truck on Friday evening, the National Transportation Safety Board said Sunday.

The truck was carrying about 7,500 gallons of anhydrous ammonia at the time, according to the Illinois Environmental Protection Agency. The agency said early estimates indicate more than half -- about 4,000 gallons -- was released.

An estimated 500 people were evacuated Friday from parts of Teutopolis, a village in south-central Illinois, following the crash but were allowed to return home Saturday.

The accident happened on US Highway 40 near Teutopolis, about 100 miles northeast of St. Louis, Missouri. The NTSB is investigating whether the tanker truck was rerouted onto the smaller highway due to an earlier crash on Interstate 70.

Crews worked overnight Friday into Saturday to patch part of the rupture on the semi-truck -- which slowed down the leak but did not stop it completely, according to Teutopolis Fire Chief Tim McMahon.

The crashed tanker was drained, patched, and taken to a secure location to be investigated by the NTSB, the state emergency management agency said. The Illinois State Police is also investigating the cause of the accident, it said.

Anhydrous ammonia "is essentially pure (over 99 percent) ammonia," says the Occupational Safety and Health Administration (OSHA).

Ammonia used in households is a diluted water solution that contains 5 to 10 percent ammonia, according to OSHA.

Ammonia in high levels can irritate and burn the skin, mouth, throat, lungs and eyes, according to the Centers for Disease Control and Prevention. Very high levels can also damage the lungs or cause death.

Symptoms of anhydrous ammonia exposure include breathing difficulty; irritation of the eyes, nose or throat; and burns or blisters.