WASHINGTON (MarketWatch) -- The Obama administration, searching for ways to respond to the worst oil spill in U.S. history, opened civil and criminal investigations of the disaster on Tuesday.

Attorney General Eric Holder announced the probes on a Tuesday visit to Louisiana, the same day that shares of BP PLC BP, -1.18% dove close to 15%. BP owns the damaged well that's been spewing oil into the Gulf since the Deepwater Horizon drilling rig exploded on April 20.

Holder didn't specify which companies may be the targets of the investigations.

But experts say that BP and other companies involved could face criminal charges as well as civil claims. Justice Department lawyers are examining BP's conduct before and after the disaster, according to The Wall Street Journal.

"We will make certain that those responsible clean up the mess they have made and restore or replace the natural resources lost or injured in this tragedy," Holder said in a statement Tuesday. "We will prosecute to the full extent any violations of the law." Read Holder's statement.

Obama briefed

Holder visited the Gulf Coast and surveyed the damage on the same day President Barack Obama met for the first time with the co-chairmen of a special commission charged with finding out the cause of the spill and preventing another similar catastrophe.

"We have an obligation to investigate what went wrong and to determine what reforms are needed so that we never have to experience a crisis like this again," the president remarked at the White House. "If the laws on our books are insufficient to prevent such a spill, the laws must change.

"If our laws were broken, leading to this death and destruction, my solemn pledge is that we will bring those responsible to justice on behalf of the victims of this catastrophe and the people of the Gulf region," Obama added.

Eleven workers died in the accident.

With the spill uncontained and still gushing oil into the Gulf, Obama had a late-morning meeting with commission co-chairmen Bob Graham, a former Democratic senator and governor of Florida; and William Reilly, who headed the Environmental Protection Agency under President George H.W. Bush. Obama said he'll appoint five other members soon.

BP's last effort to stop the leak, called the "top kill," failed over the weekend, and now the disaster could stretch into August, when relief wells will be completed.

Holder was in Louisiana on Tuesday to meet with state attorneys general and U.S. attorneys from the region. Last month, Holder said the administration would uphold the law as it relates to the spill.

The size of the spill already has surpassed the 1989 Exxon Valdez spill, for which oil giant Exxon Mobil Corp. XOM, +0.14% agreed to pay $100 million in criminal penalties alone. Exxon pleaded guilty to violating the Clean Water Act, the Refuse Act and the Migratory Bird Treaty Act.

Obama said he authorized the commission to hold public hearings and request information from government agencies, BP, Transocean Ltd., Halliburton Co. and others. Transocean RIG, -4.26% owned the Deepwater Horizon rig and Halliburton HAL, -0.16% installed some of the equipment on it.

The disaster has taken a $100 billion bite out of the market capitalization of the five key companies connected to the spill over the last six weeks. Read about spill's effect on energy shares.