SEATTLE (AP) -- Tesoro Corp. says it is under investigation by the Environment Protection Agency's criminal division for the April 2 explosion and fire that killed seven people at the oil company's Anacortes, Wash., refinery.

The Seattle Times said San Antonio-based Tesoro revealed the investigation in a quarterly earnings report filed late last week. The company's previous filings with the Securities and Exchange Commission have said the EPA was looking into the accident but didn't make clear whether it was by the agency's civil or criminal investigative branch, the newspaper reported.

Last month, the Washington Department of Labor and Industries fined Tesoro $2.39 million, the largest amount in the agency's history, after a six-month investigation into state workplace safety and health regulations. It cited the company with 39 willful and five serious violations.

The agency said Tesoro failed to check for cracks that had developed in a nearly 40-year-old heat exchanger that blew apart and killed seven workers at the refinery about 70 miles north of Seattle. The cracks likely developed over the years, it said.

Tesoro has appealed the fine.

The U.S. Chemical Safety Board also is investigating the incident. It's report isn't expected until early next year.

Tesoro spokesman Mike Marcy told The Times it is working with all the investigating agencies.

"We have provided these regulatory bodies with all requested information and are cooperatively keeping them in the loop going forward," Marcy said.