The report includes an interview with a 17-year-old girl who said she had no idea she had forced the tanker off the road.
The crash happened last September on Highway 40 in Effingham County, near Teutopolis.
Investigators showed the girl dashcam video from the tanker. She acknowledged she was at the wheel when her minivan passed the tanker then cut it off to avoid a collision with oncoming traffic.
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The tanker hit a trailer on the side of the highway.
That caused an anhydrous ammonia leak that killed two nearby drivers and a father and his two children in their home.