RENO, Nev. (AP) -- 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.

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. The group 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."

There's not much Obama can do to affect the price of gasoline in short term, something he acknowledged in his remarks. Gas prices have risen steadily as a result of tensions in the Middle East and northern Africa and rising demand from China and other emerging economies.

Given that no evidence has yet surfaced of actual fraud or price manipulation in oil markets, Obama's remarks appeared, at least in part, as more of an attempt to assuage public anger over rising gas prices.

Other presidents have also launched similar inquiries at times of rising oil prices and widespread public suspicions of market manipulation by the oil industry or by speculators.

In an Associated Press-Gfk poll last month, 51 percent of adults said they thought recent increases in gas prices were due to "oil companies that want to boost profits" rather than changes in the global oil market. Nine percent said higher prices stemmed from a combination of both, 37 percent from changes in the market.

Obama renewed his proposal to end roughly $4 billion annually in various government subsidies to oil and gas companies "at a time when they're making record profits and you're paying near record prices at the pump. It has to stop."

Asked by a member of the audience about prospects for advancements in clean energy, Obama predicted that, with time, prices of now-expensive devices such as electric cars would come down.

"Having a flat-screen TV used to be a big deal," Obama said. But he said now such TVs are commonplace because prices have dropped so much.

While acknowledging he doesn't spend much time these days behind the wheel, Obama said, "I've been in one of these Chevy Volts. This is a nice car. It drives well."

He said when such vehicles become more affordable, "those of you out there driving that 8-mile-a-gallon SUV" should consider a purchase. Otherwise, by buying gasoline that likely came from imported oil, Americans "are putting money in the pockets of people who do not like us at all," he said.

Earlier, he told supporters in San Francisco that he is pressing ahead with his agenda in a difficult political environment and that "change turned out to be a lot tougher than expected."

Obama addressed about 200 people who paid up to $35,800 apiece for the fundraiser at San Francisco's St. Regis Hotel, the first of four fundraisers of the day. The other three were scheduled in Los Angeles.

Obama was interrupted by a small group among the paying guests who protested the detention of Bradley Manning, an Army private accused of leaking secret documents to the WikiLeaks website.

"We paid our dues, where's our change?" the protesters sang to the president.

"We'll vote for you in 2012, yes that's true. Look at the Republicans - what else can we do?"

Obama paused while security removed some of the protesters, then joked, "That's a nice song. You guys have much better voices than I do."

Manning, suspected of illegally passing U.S. government secrets to the WikiLeaks while serving as an Army intelligence analyst in Iraq, was transferred this week to an Army prison in Kansas from the Marine brig in Quantico, Va., where he has spent the last nine months.

Between his California events, Obama went to the Electra Therm Co. in Reno, speaking in front of a machine that produces renewable energy from low-temperature heat waste.

Obama's West Coast visit - his most extensive travel since announcing his re-election bid two weeks ago - offered a glimpse of how he will seek to re-energize the independents and first-time voters who carried him to victory in 2008. Obama argues that more work must be done to make the vision of America he promised a reality and that he is the only one who can see those hopes through.