“What is undoubtedly true is that we did not have the tools you would want in your tool kit,” he told The Financial Times in an interview published Thursday.

It is “an entirely fair criticism,” he said, to fault the company for not being fully prepared for a leak one mile below the surface of the gulf. In the wake of the disastrous 1989 Exxon Valdez spill in Alaska, he said, BP and other oil companies prepared intricate plans to contain oil on the surface of the water. But BP did not have all the equipment it should have had to stanch a deepwater leak like the one in the gulf. It will have to find ways to manage such “low-probability, high-impact” risks in the future, he said. The announcement that the riser had been cut was a rare glimmer of positive news The operation had hit a snag on Wednesday when the saw became stuck. The riser it was trying to cut is a mile-long pipe that once ran from the wellhead up to the drilling rig, and now lies snaking along the seabed.

The technician involved in the effort said the wire saw had cut less than halfway through the riser when it stopped being effective. The saw was freed later Wednesday afternoon, but officials then decided to use the shear instead. The shear, about 20 feet long and nearly 10 feet high, was used to make an earlier cut in the riser about 50 feet away.

Because the cut it made on Thursday was not as clean as the wire saw would have made, the containment cap will have to be modified. The technician said he still expected the cap to be in place by Thursday. The technician, who spoke on the condition of anonymity because he was not authorized to comment on the work, said it appeared that the saw had been dulled by material inside the riser  including, perhaps, some of the objects pumped into the well during the failed “top kill" procedure last week.

The gushing well, 5,000 feet below the gulf’s surface, might not be stanched permanently until mid-August, when BP is expected to complete the first of two relief wells that will allow the stricken well to be plugged with cement. Admiral Allen said on Thursday that that effort was ahead of schedule.

Admiral Allen also announced that he had approved a request by the governor of Louisiana, Bobby Jindal, to build five additional sand-barrier islands along the coast to protect sensitive inlets from the spreading oil, and that BP would pay for them. BP said on Thursday that the project would cost about $360 million, on top of the $990 million it has already spent so far on the cleanup and on paying claims made by people and companies who have been hurt by the spill.

Slicks of oil were edging closer to the white sand beaches of the Florida Panhandle on Thursday, and emergency workers hurried to link up chains of protective boom, stretching for miles, that could prevent much of the oil from reaching the shore.

The National Center for Atmospheric Research, a nongovernmental research institute based in Colorado, said models showed Thursday that oil could enter the Gulf’s loop current, go around the tip of Florida and as far north as Cape Hatteras, N.C. According to researchers, oil could threaten East Coast beaches by early July, but they cautioned the models were not a forecast.