Evacuation orders remained in place for about 50,000 people in Garden Grove, California, located south of Los Angeles.
Federal investigators' final report includes an interview with a 17-year-old girl who said she had no idea she had forced the tanker off the road.
Authorities confirmed the leak was styrene, a flammable liquid used to make plastics and rubber, according to the National Institute of Environmental Health Sciences.
Customers and staff quickly evacuated the restaurant after the hazmat situation was reported.
The family of an 18-year-old woman who was critically injured in a tanker truck crash in central Illinois has retained Ben Crump as their attorney.
An attempt to pass a tanker may have led to the deadly chemical crash, an official said.