Five people are dead and hundreds of nearby residents were evacuated after a semi-truck carrying a toxic substance overturned in rural Illinois Friday night, officials said.

The incident occurred around 8:40 p.m. local time on U.S. Highway 40 near the village of Teutopolis, authorities said.

A semi-truck transporting anhydrous ammonia rolled over, causing a "large plume cloud of anhydrous ammonia on the roadway that caused terribly dangerous air conditions in the northeast area," Effingham County Sheriff Paul Kuhns told reporters during a Saturday press briefing.

Five fatalities and "multiple injuries" have been reported, according to the Illinois Environmental Protection Agency. Several vehicles were involved in the crash, state police said.

The semi-truck was carrying approximately 7,500 gallons of anhydrous ammonia at the time of the accident, according to the Illinois Environmental Protection Agency. An estimated 4,000 gallons have been released, it said.

The accident scene was "large" and "complicated," Kuhns said.

"We're still dealing with the crash and the emergency caused by the anhydrous ammonia spill," the sheriff said.

About 500 people have been evacuated in the wake of the crash, with the evacuation zone approximately 2 square miles on the east side of Teutopolis, authorities said. There is no update at this time on when residents will be able to return to their homes.

The ruptured area of the tanker has been patched, which "slowed it down" but did not stop the leak, Teutopolis Fire Protection District Chief Tim McMahon said.

"A contractor hired by the responsible tanker truck is en route to the scene to review the situation and advise on the best plan to safely empty the tanker," the Illinois Environmental Protection Agency said in a statement.

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.

Drivers are still being kept away from the area amid the cleanup.

The cause of the crash remains under investigation, Acting Lt. Bruce Wagner of the Illinois State Police said.

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

Representatives from the U.S. Environmental Protection Agency are also at the scene.

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