By injecting solid objects as well as heavy drilling fluid into the stricken well leaking oil into the Gulf of Mexico overnight, engineers appeared to have stemmed the flow of oil, Adm. Thad W. Allen of the Coast Guard, the leader of the government effort, said on Friday morning. But he stressed that the next 12 to 18 hours will be "very critical" in permanently stanching what is already the worst oil spill in United States history.

Admiral Allen, who spoke on ABC's Good Morning America show, said the biggest challenge will be to sustain the "top kill" effort, which involves pumping material into the well to counteract the upward pressure of the gushing oil so that the well can be sealed."They've been able to push the hydrocarbons and the oil down with the mud," he said, referring to the heavy drilling fluid. "The real challenge is to put enough mud into the well to keep the pressure where they can put a cement plug over the top."

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