U.S. regulators are urging railroads to make dramatic operating changes, including how they deal with wheel defects, saying a wheel problem may have caused the fiery oil-train derailment in Illinois last month.

Despite multiple warning signs, a train carrying crude oil from North Dakota to Philadelphia continued to travel on a potentially faulty wheel, according to a preliminary federal investigation.

Twenty-one cars of a BNSF Railway Co. oil train derailed near Galena, Ill., 160 miles west of Chicago. Several cars ruptured during the accident and the oil inside caught fire, generating large explosions.

On Friday, the Federal Railroad Administration issued a safety advisory pointing to a broken wheel and telling railroads to act more aggressively to fix similar defects found on other trains.

According to investigators, a trackside device flagged the oil train’s defective wheel about 130 miles before the derailment. A month before the accident, other similar devices registered a reading on this railcar’s wheel at a level that indicated there was a flat spot that made it “condemnable,” according to the safety advisory.