LOS ANGELES - US President Barack Obama said Thursday that the Justice Department will try to "root out" cases of fraud or manipulation in oil markets, even as Attorney General Eric Holder suggested a variety of legal reasons may be behind gasoline's surge to $4 a gallon.

"We are going to make sure that no one is taking advantage of the American people for their own short-term gain," Obama said at a town-hall style meeting at a renewable energy plant in Reno, Nev.

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The town hall was sandwiched in between Obama's four fundraising events in California on Thursday - one in San Francisco and three in Los Angeles. The president was holding six fundraisers over the course of his three-day West Coast trip, aimed at high-dollar donors and young people, both of whom will be integral to a campaign that could set fundraising records.

"This is going to just as hard, if not harder, than 2008," he said of his re-election bid during a small fundraiser at Sony Pictures Studios.

With the 2012 campaign in mind, the White House is anxious to show the public it's taking action to address rising gasoline prices. The national average price for a gallon of regular gasoline was $3.84 on Thursday, about 30 cents higher than a month ago and almost a dollar higher than a year ago.

Obama, decrying such levels as yet another hardship "at a time when things were already pretty tough," said Holder was forming the Financial Fraud Enforcement Working Group.

The task force will focus some of its investigation on "the role of traders and speculators" in the oil-price surge, Obama said, and will include several Cabinet department officials, federal regulators and the National Association of Attorneys General.

In Washington, Holder said he would press ahead with the investigation, even though he did not cite any current evidence of intentional manipulation of oil and gas prices or fraud.

"Based upon our work and research to date, it is evident that there are regional differences in gasoline prices, as well as differences in the statutory and other legal tools at the government's disposal," Holder said in a memo accompanying a statement announcing the task force. "It is also clear that there are lawful reasons for increases in gas prices, given supply and demand."

"Nonetheless, where consumers are harmed by unlawful conduct that has the effect of increasing gas prices, state and federal authorities will take swift action," Holder said.

He promised to "be vigilant in monitoring the oil and gas markets for any wrongdoing so that consumers can be confident they are not paying higher prices as a result of illegal activity."