Authorities resumed their investigation Sunday of a central Illinois truck crash that killed five people, injured five others and prompted an evacuation for hundreds of residents after a toxic chemical leak.

The tanker truck, which was carrying caustic anhydrous ammonia, was drained and moved to a “secure location” for a National Transportation Safety Board investigation, authorities said late Saturday as residents were allowed to return to their homes in the Teutopolis area.

“Testing has indicated that the danger from the anhydrous ammonia has dissipated,” Teutopolis Assistant Fire Chief Joe Holomy said late Saturday.

A semitruck carrying caustic anhydrous ammonia toppled about 9:25 p.m. Friday in Teutopolis, spilling more than half its 7,500-gallon load, according to the Illinois Environmental Protection Agency. Teutopolis is about 110 miles northeast of St. Louis.

Effingham County Coroner Kim Rhodes said the five dead included three from the same family: one adult and two children under 12. The other two were adult motorists from out of state, Rhodes said. Additionally, five people were airlifted to hospitals, their conditions unknown.

Names of the victims were not released, nor would authorities discuss causes of death.

About 500 residents within a 1-mile radius of the crash site were evacuated after the accident, including northeastern parts of Teutopolis.

Emergency crews worked overnight after the accident on Friday trying to control the plume from the leak and struggled to get near the crash site.

“We have a lot of brave firemen, EMT, hazmat specialists, police officers that are working on this scene as we speak,” Effingham County Sheriff Paul Kuhns said at a Saturday morning news conference.

Private and federal environmental contractors were summoned to recommend a cleanup procedure in Teutopolis, a town of 1,600 people.

The accident caused “a large plume, cloud of anhydrous ammonia on the roadway that caused terribly dangerous air conditions in the northeast area of Teutopolis,” Kuhns said. “Because of these conditions, the emergency responders had to wait. They had to mitigate the conditions before they could really get to work on it, and it was a fairly large area.”

Although not strong, crews working overnight struggled against shifting wind.

“The wind changed three or four different times on us,” said Tim McMahon, chief of the Teutopolis Fire Protection District. “That’s another reason we got crews out in different places, reporting back on which way the wind’s going.”

Traffic, including the tanker, was pushed onto U.S. 40, which bisects Teutopolis, earlier Friday because of another truck crash on Interstate 70.

Phillip Hartke, 75, who lives in Teutopolis but farms with his son outside of town, said U.S. 40 was jammed after the I-70 closure. Hartke finished harvesting corn about 9:30 p.m. Driving home, as he neared the center of town, he could smell anhydrous ammonia. When he reached U.S. 40, emergency vehicles swarmed the area.

“Firefighters advised us right there: ‘Evacuate to the west,’” Hartke said.

Hartke estimated 85% of Teutopolis was subject to the evacuation. He and his wife were staying with his son. Such familial ties should serve most evacuees well.

“‘T-Town’ is a tight-knit community,” Hartke said. “Many people have sons and daughters, aunts and uncles within five or six miles of town.”

Anhydrous ammonia is used by farmers to add nitrogen fertilizer to the soil and as a refrigerant in the cooling systems of large buildings such as warehouses and factories. According to the American Chemical Society, it is carried around the United States by pipeline, trucks and trains.

In 2019, dozens of people were sickened in suburban Chicago after the valves were left open on tanks of anhydrous ammonia being transported from a farm in Wisconsin to one in Illinois, creating a toxic gas cloud and sickening dozens of people. Seven people were initially hospitalized in critical condition after a leaking anhydrous ammonia tank pulled by a tractor released the plume over Beach Park.

And in 2002, a train derailment released anhydrous ammonia in Minot, North Dakota, killing one man, and hundreds of other people reported injuries including burns and breathing problems.

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

In addition to having a commercial driver’s license, the person behind the wheel of a toxic-substance tanker must study further and successfully complete a test for a hazardous material endorsement, said Don Schaefer, CEO of the Mid-West Truckers Association.

“Once you get that endorsement, there are no restrictions — unless otherwise posted — on hauling hazardous materials on a public highway,” Schaefer said. “But you’re subject to higher scrutiny.”

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Slevin reported from Denver. Associated Press writers Pamela Sampson in Atlanta and Kathleen Foody in Chicago contributed.