The U.S. has opened a criminal investigation into the Gulf of Mexico oil spill, Attorney General Eric Holder said.

“We will prosecute to the fullest extent of the law anyone who has violated the law,” said Mr. Holder, who was in Louisiana on Tuesday meeting with state attorneys general from the region and touring the areas affected by the huge spill from a well owned by British oil giant BP PLC.

Mr. Holder wouldn’t specify who exactly was under investigation, because he said authorities aren’t “clear on who should ultimately be held liable.” Also he said “we don’t want to cast aspersions.”

The announcement sent shares tumbling. BP shares, already down on the weekend news that an effort to plug the well had failed, fell further in New York to close down almost 15%. Shares of other energy companies also fell amid fears that oil companies might have to pay out huge amounts in fines, or see their operations affected by new curbs on offshore drilling.

BP said it would cooperate “with any inquiries that the Department of Justice undertakes, just as we are doing in response to the other inquiries that are already ongoing,” a spokesman said.

The attorney general said there was a range of possible violations under a number of statutes, including the Clean Water Act, the Oil Pollution Act of 1990 and the Endangered Species Act. He said charges could include everything ranging from “false statements” to “the way in which certain entities conducted themselves.”

Mr. Holder said his department had instructed all relevant parties to preserve documents.

“If we find evidence of illegal behavior we will be extremely forceful in our response,” he said, adding that “we have what we think is a sufficient basis for us to have begun a criminal investigation.”

The Justice Department is walking a fine line since potential parties under investigation are crucial in the cleanup efforts. Mr. Holder said, however, that he believed parties that might be probed—such as BP—had an incentive to redouble their cleanup efforts since they would likely want to “mitigate whatever damages they have caused.”

Mr. Holder also said multiple times that the responsible parties, not American taxpayers, would ultimately be on the hook for cleanup costs.

In another move by the government to ramp up its investigations into the out-of-control well, which has spewed more oil into the Gulf than the 1989 crash of the Exxon Valdez in Alaska to become the worst oil spill in U.S. history, President Barack Obama said Tuesday he wanted a new oil-spill commission to thoroughly investigate the matter.

“In doing this work, they have my full support to follow the facts wherever they may lead,” Pres. Obama said in the White House Rose Garden after emerging from a meeting with the co-chairmen of the National Commission on the BP Deepwater Horizon Oil Spill.

He said if laws were broken, justice would be done. He also said the federal government was monitoring the situation minute-by-minute.

Pres. Obama said the commission would report back to him in six months on the causes of the spill, the government’s response and what changes were needed to oil regulations to prevent a repeat of the disaster.

“Only then can we be assured deep-water drilling can be done safely,” he said after noting that he had halted exploratory deep-water drilling along the Gulf coast for six months, while the commission conducts its investigation.

Pres. Obama traveled to Louisiana on Friday to survey the disaster and the federal government’s cleanup efforts amid growing criticism the government hasn’t done enough.

He said over the weekend that it is an outrage that the mile-deep well continued to leak roughly half a million gallons of oil daily and that attempts to plug the leak continue to fail.

Frustrations over the disaster appear to grow daily as it persists, despite proclamations from the government and BP that the best and brightest minds are working to solve the crisis.

The Obama administration has taken a tough approach to dealing with BP, and over the weekend officials clashed with the British oil giant.

Administration officials said the latest attempt to stop the leak could increase the flow by 20% temporarily despite suggestions from BP that the new move posed little risk.