At least six people were killed and more than 30 people were injured in crashes caused by excessive winds that blew dust onto Interstate 55 in Farmersville, Ill., on Monday morning, leading to zero visibility conditions, the Illinois State Police said.The crashes, which took place just before 11 a.m. in a rural area south of Springfield, involved about 20 commercial vehicles and 40 to 60 cars, including two tractor-trailers that caught fire, the police said in a statement.