WASHINGTON - Striking an aggressive tone as a ruptured well spewed oil into the Gulf of Mexico for a sixth week, the Obama administration said Tuesday it would launch a criminal probe into the drilling-rig explosion that has led to the biggest environmental disaster in U.S. history.

"If we find evidence of illegal behavior, we will be extremely forceful in our response," Attorney General Eric H. Holder Jr. said in New Orleans after viewing damage caused by the unchecked spill. Holder said he believed there was "sufficient evidence" to warrant a criminal inquiry.

BP undertook yet another effort to stop the gusher. It planned to trim a broken pipe and put a dome on it, a maneuver that could at least temporarily worsen the flow and was not guaranteed to succeed.

President Obama met Tuesday with members of a commission he formed to recommend ways of preventing a similar disaster.

"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," Obama said.

He appeared in the Rose Garden with the commission's cochairmen, former Democratic Sen. Bob Graham of Florida and William K. Reilly, who headed the Environmental Protection Agency under Republican President George H. W. Bush.

Obama ordered the two "to follow the facts wherever they lead, without fear or favor." He said he expected a report from them in six months.

Holder said Justice Department lawyers were examining whether the companies that owned or operated the Deepwater Horizon rig in the gulf, including BP P.L.C. and Transocean Ltd., had violated federal statutes that call for criminal and civil penalties, including the Clean Water Act and the Endangered Species Act.

"There are a wide range of possible violations under these statutes, and we will closely examine the actions of those involved with the spill," said Holder, who did not identify the companies targeted by the inquiry by name and did not speculate about the range of criminal penalties they might face.

BP officials in Houston said the company would cooperate with the federal investigation.

Like a deli slicer

After weeks of failures to block the well or divert the oil, BP planned to use robotic machines to carve into the twisted appendages of the crippled well.

The latest attempt involves using tools resembling an oversize deli slicer and garden shears to break away the broken riser pipe so engineers can then position a cap over the well's opening.

Even if it succeeds, it will temporarily increase the flow of an already massive leak by 20 percent - at least 100,000 gallons more a day. And it is far from certain that BP will be able to cap a well that one expert compared to an out-of-control fire hydrant.

"It is an engineer's nightmare," said Ed Overton, a Louisiana State University professor of environmental sciences. "They're trying to fit a 21-inch cap over a 20-inch pipe a mile away. That's just horrendously hard to do. It's not like you and I standing on the ground pushing - they're using little robots to do this."

The operation has never been performed in such deep water, and is similar to an earlier failed attempt that used a larger cap that quickly froze up.

No guarantees

BP officials said they were applying lessons learned from the earlier effort and planned to pump warm water through pipes into the smaller dome to prevent any icing problems.

"If all goes as planned, within about 24 hours we could have this contained," BP's chief operating officer, Doug Suttles, said Tuesday after touring a temporary housing facility set up for cleanup workers in Grand Isle, La. "But we can't guarantee success."

The solution that offers the best prospect of halting the ceaseless flow of oil involves the digging of relief wells, an effort unlikely to be completed until August.

As for the inquiries, Suttles said on Fox News that BP would let them run their course, "and they'll find what they find and we'll deal with that when they find it."

BP paid a $50 million fine after a fire and explosion at a Texas City, Texas, oil refinery killed 15 workers and injured 170 in 2005. ExxonMobil Corp. paid a $125 million criminal fine for the Valdez spill in Alaska.

The Deepwater rig explosion April 20 killed 11 workers and has pumped an estimated 21 million to 44 million gallons of oil into the gulf - eclipsing the 11 million gallons that leaked during the 1989 Exxon Valdez disaster.

On Tuesday, oil reached Mississippi for the first time after already reaching the shores of Louisiana and Alabama.

Officials in Florida said Tuesday that an oil sheen was about nine miles off the state's coast, and that it could hit the white sands of Pensacola Beach as soon as Wednesday.

Escambia County, Fla., officials started putting out booms Tuesday and making other plans for the arrival of the oil.

With mystery swirling over how much oil may be lurking beneath the surface of the gulf, a research vessel was to leave Wednesday on a nine-day mission aimed at finding and studying a potentially toxic stew that oceanographers fear could be catastrophic for marine life.

The National Oceanic and Atmospheric Administration's Thomas Jefferson, one of the most technologically advanced vessels for finding hazards on the seafloor, has been diverted from a recent trip to map the ocean floor off Galveston, Texas, to the belching Deepwater oil leak.

Some researchers have found what they say are vast plumes of oil suspended beneath the gulf's surface, though BP, owner of the leaking well, has disputed those reports. Members of Congress and other researchers have been pressing the White House for weeks to do more to determine how much oil is suspended under the surface.