Venice, Louisiana (CNN) — BP started a “top kill” procedure Wednesday afternoon in an attempt to contain oil gushing into the Gulf of Mexico, the company said.

At 1 p.m. CT (2 p.m. ET), the oil giant began pumping 50,000 pounds of thick, viscous fluid twice the density of water into the site of the leak to stop the oil flow, it said. If all goes according to plan, the well then could be sealed shut with cement.

BP Chief Executive Officer Tony Hayward said earlier the procedure has a 60 percent to 70 percent chance of succeeding.

BP conducted hours of diagnosic testing before starting, and Coast Guard Rear Adm. Mary Landry, the federal on-scene coordinator for the oil spill response effort, granted the final authorization for the company to move forward with the much-awaited operation.

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“If it’s successful, and there are no guarantees, it should greatly reduce or eliminate the flow of oil now streaming into the Gulf from the seafloor,” Obama said after discussing the top kill procedure with Energy Secretary Steven Chu.

“And if it’s not, there are other approaches that may be viable.”

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