BP early Wednesday used robot submarines and a complex maneuver to stop the massive flow of crude oil spilling into the Gulf of Mexico.

The company appeared to be making its second cut into the undersea well's riser pipe, the initial steps toward placing a cap over the well that has spewed hundreds of thousands of gallons of oil into the Gulf of Mexico every day since late April.

Meanwhile, rust-colored oil washed ashore on barrier islands off Alabama and Mississippi on Tuesday as a beleaguered BP tried to stop the continued flow of the largest spill in U.S. history.

Doug Suttles, the company's chief operating officer, told CNN's "John King USA" Tuesday night that the latest procedure should collect the "vast majority" of the oil if it succeeds.

"We'll be putting the cap assembly, loading that out and sending it to the sea bed later tonight," Suttles said. "We should be able to install this tomorrow. And hopefully by late tomorrow or Thursday, we should have this thing operating."

But the operation carries the risk that the flow of crude from the ruptured well could increase by up to 20 percent once the damaged riser is cut away. The job already has been complicated by pipework around the well that has had to be removed before massive metal shears could be brought to bear, Suttles said. And the gusher may not be shut down until August, when BP expects to complete relief wells that will take the pressure off the one now spewing into the Gulf.

The 5,000-foot-deep well erupted after an explosion and fire aboard the Deepwater Horizon drilling rig on April 20. The rig sank two days later, taking 11 men with it and leaving up to 19,000 barrels (798,000 gallons) of oil pouring into the Gulf, according to federal estimates.