The National Transportation Safety Board has opened a safety investigation into the collision that caused a cargo tank carrying 7,500 gallons of anhydrous ammonia to overturn in Illinois on Friday.
The cargo tank was in an accident that involved "multiple" vehicles about a half-mile east of Teutopolis, Illinois, on U.S. Highway 40 at about 9:25 p.m. local time, authorities said in a statement.
Five fatalities were reported, and six other people were hospitalized. The identities of the victims have not been released.
The impact of the accident caused the cargo tank to start leaking, which prompted the evacuation of 500 people, according to a news release from the Illinois Environmental Protection Agency.
At a press conference Sunday afternoon, NTSB board member Tom Chapman said a preliminary investigation determined the accident was caused when the driver of a semi-truck pulled to the right of the road in reaction to another vehicle.
The truck rolled over, compromising the cargo tank carrying the ammonia, Chapman said.
The cargo tank was punctured after it collided with a parked utility trailer, causing about half of the tank's 7,500 gallons of anhydrous ammonia to leak, according to Chapman.
"We will not be determining the probable cause of the crash while we were on scene, nor will we speculate about the cause," Chapman said.
The investigative team will be on the scene for four to six days and a preliminary report is expected to be published in about 30 days, according to Chapman. Final reports take from 12-24 months to complete.
Chapman encouraged witnesses or anyone with relevant information regarding the collision to contact the NTSB.
Effingham County Sheriff Paul Kuhns earlier told reporters that there was a "large plume" from the ammonia leak. Because the leak caused "terribly dangerous air conditions in the northeast area of Teutopolis," an evacuation was ordered within an approximate one-mile radius of the crash, Kuhns said. Evacuation orders for some areas were lifted Saturday evening, Effingham County reported.
Officials declined to comment on what caused the deaths.
The air conditions meant emergency responders had to "wait" and "mitigate the conditions" before they could "get actual access" to the crash site, Kuhns said. Kuhns said the crash site covered a "fairly large area." The highway between Teutopolis and Montrose is closed.
"We have a lot of brave firemen, EMT, hazmat specialists, police officers that are working on the scene as we speak," said Kuhns.
The NTSB said in a statement shared Saturday on X, the social media site formerly known as Twitter, that it will investigate the crash in coordination with the Illinois State Police and the Effingham County Sheriff's Department.
Inhalation of anhydrous ammonia, which is toxic, can be fatal at high concentrations. The ammonia can also burn the skin and eyes, and cause severe respiratory injuries. In the news conference, one official called the substance "terrible."
Teutopolis is a town of just over 1,600 people about 92 miles southeast of Springfield, Illinois, the state's capital.
The Associated Press contributed reporting.
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