With a green light from the U.S. Coast Guard Wednesday, BP began trying to plug the oil leak with a technique that could take up to Friday to finish.
"We started pumping the drilling muds for the so-called 'top kill' operation," said BP spokesman David Nicholas.
"This operation is now under way," he added. "It is an operation that is going to take an indeterminate and undefined length of time. it could potentially be up to two days."
Earlier, the federal on-scene coordinator, Coast Guard Rear Adm. Mary Landry gave approval to BP's latest attempt to cap the well, the final authorization the company needed to start pumping heavy drilling mud and cement into the breach.
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President Barack Obama is scheduled to visit Louisiana Friday for the second time since the spill began more than a month ago, amid criticism of BP's and the government's response the the disaster.
(Rosenberg reports for the Miami Herald. This story will be updated.)