A fiery oil train derailment that forced the evacuation of hundreds of people in West Virginia last February was caused by a rail defect that railroad inspectors from CSX Corp missed twice in the preceding months, U.S. regulators said on Oct. 9.

Twenty-seven of the train's 109 cars derailed near Mount Carbon, West Virginia, on Feb. 16. At least nine cars caught fire and burned for days, strengthening support for tougher oil train safety standards for trains carrying U.S. crude from North Dakota's Bakken region. Tighter federal regulations were published in May.

The Federal Railroad Administration fined CSX and its contractor, Sperry Rail Service, $25,000 each for failure to properly verify a potential rail defect. An FRA spokesman said the sum is the maximum allowed under federal law for such a violation.

FRA also called for more detailed inspections across the industry and stronger training for inspection vehicle operator. It also said it would explore the need for new rail standards and slower train speeds under certain conditions.

The U.S. railroad regulatory agency said a verticle split rail head defect caused the rail to break as the CSX train passed. Inspectors failed to notice the fault during inspections in December 2014 and January 2015.