Washington ? The Federal Trade Commission has begun a formal investigation of the proposed shutdown of a Shell Oil Co. refinery in California to determine possible antitrust violations, a senior FTC official said Wednesday.

The refinery near Bakersfield, Calf., has been the subject of controversy as Shell officials plan to close the facility in November. The oil company said the refinery was being shut down because of a decline in oil production in the region.

But critics maintain that the shutdown is part of a strategy to continue tight oil markets and increase gasoline prices, which already are the highest in the country in California.

William Kovacic, the FTC’s general counsel, said subpoenas already had been served as part of the investigation and that the probe is being viewed as a top priority of the agency.

“We regard this as a matter of particular urgency and importance,” he said.

Kovacic disclosed the investigation at a hearing into high gasoline prices before a House Government Reform subcommittee, where lawmakers questioned why the FTC was not taking a more aggressive role in examining the Bakersfield refinery issue.