LOS ANGELES (CBSLA.comAP) — California Attorney General Kamala Harris has issued subpoenas to several oil refiners to learn how they set gasoline prices, which are consistently higher in California than in most other states.

Chevron Corp., Exxon Mobil Corp., Valero Energy Corp. and Tesoro Corp. confirmed on Thursday that they have received subpoenas in recent weeks.

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“Oil refiners have not competed to bring gasoline into this market. They’ve cheated to short this market and drive up prices,” Consumer Watchdog President Jamie Court said at a news conference Thursday.

The attorney general is making a sweeping request for information about gasoline supplies, pricing, and maintenance shutdowns that can temporarily create shortages and increase prices, according to people familiar with the investigation. The people spoke on condition of anonymity because they were not authorized to discuss details of the subpoenas.

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Harris is a Democrat who is running for the U.S. Senate. Kristin Ford, a spokeswoman for Harris, declined to comment on whether her office was investigating.

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