On Thursday, California Attorney General, Jerry Brown subpoenaed financial records and other documents from the state's seven biggest health insurance companies as part of a broad investigation of the industry.

The seven companies covered by the subpoenas -- Aetna Health, Anthem Blue Cross, CIGNA, Health Net, Blue Shield of California, Kaiser Permanente and PacifiCare - are given 30 days to submit their records.

Brown said that his office was looking into "possibly illegal practices" among state health insurers and is "very concerned that some of them are unjustly raising premiums and denying payment of legitimate claims".

Meanwhile, Anthem Blue Cross, California's biggest health insurer, said earlier this week that it would go ahead with plans to raise its premiums for some individuals by up to 39%, in spite of criticism from the Obama administration.

WellPoint Inc., Anthem's parent company, appeared on Capitol Hill to protect its rates, declining charges that it was raising premiums to "squeeze" consumers in the name of profit.

Industry executives say increasing premiums reflect soaring costs of delivering medical care and mass departure of many healthy consumers from the ranks of policyholders.

"Our best attorneys are going to get to the bottom of this, and where we find violations of California's unfair business laws, we intend to stop them", Brown said.