President Obama plans to meet Tuesday with the cochairmen of an independent commission investigating BP's catastrophic spill in the Gulf of Mexico.

A senior administration official said Obama would meet at the White House with Bob Graham, a former Florida governor and former U.S. senator, and William K. Reilly, a former head of the Environmental Protection Agency. It will be Obama's first meeting with the two since naming the commission less than two weeks ago.

The session comes three days after BP said its latest attempt to stop the oil spewing out of a broken well 5,000 feet underwater had failed.

On Monday, BP's robots wielded clamps and hauled machinery in a slow-motion ballet a mile below the surface of the gulf, preparing the next risky maneuver to stanch the gush of oil from its renegade well.

The initial step to sever the crumpled and ruptured broken riser pipe that is spewing as much as 798,000 gallons of oil a day was expected late Monday or Tuesday.

But BP spokesman Graham MacEwen said the company could not predict when it would be ready to cut the ragged pipe, a necessary operation to enable the snug fit of a cap that would funnel oil to a ship at the surface.

BP has predicted the new effort could capture much of the oil, but if it failed, it could boost the flow by removing any resistance the pinched and bent pipe may have created.

Ultimately, the catastrophic leak can be halted only when a relief well - two are being drilled - intercepts the existing well in August at the earliest to enable a new cementing job.

The robots Monday were "preparing the area, making smaller cuts or trimming operations," MacEwen said. The company's website, bp.com, would continue to stream live video of the maneuver, he said.

As the struggle to reduce the flow continued, the National Oceanic and Atmospheric Administration predicted that moderate winds from the south and southwest this week could move oil slicks closer to the Mississippi and Alabama coasts by Wednesday.

Nearly six weeks have elapsed since the April 20 blowout that killed 11 men aboard the Deepwater Horizon rig and started the largest oil spill in U.S history. About 120 miles of Louisiana coastline have been contaminated by slicks and tar balls, and a quarter of federal waters in the region remain closed to fishing.

Controversy arose Monday over BP chief executive Tony Hayward's questioning over the weekend of statements from two scientific research teams that vast underwater plumes of hydrocarbons have been detected in a wide area of the gulf.

BP found "no evidence" of plumes in its own tests, he said.

Rep. Edward J. Markey (D., Mass.), chairman of a subcommittee investigating the spill, mocked Hayward's assertions, saying: "BP in this instance means 'Blind to Plumes.' "

He released a letter sent to BP America president Lamar McKay asking for "copies of all measurements, calculations or other supporting materials on which Mr. Hayward based his statements regarding the existence of subsurface plumes of oil."

Obama still must name five members of the commission, which will investigate such issues as what caused the spill, the safety of offshore drilling, and operations at the federal agency that grants drilling rights.

The administration said Obama was not expected to announce those names Tuesday.

Attorney General Eric H. Holder Jr. will tour the spill area Tuesday and meet with federal prosecutors and state attorneys general, the Justice Department announced Monday.

The department last week told BP, the owner of the well, in addition to rig operator Transocean Ltd. and cementing contractor Halliburton Co., to preserve all paperwork connected to the accident, in what appeared to be the first step toward a criminal investigation.

This article contains information from the Associated Press.