ROBERT, La. -- BP says it has resumed pumping heavy drilling mud into a gushing well in the Gulf of Mexico in a bid to stop the flow of oil.

The operation, known as a 'top kill,' is considered the best chance to halt the worst oil spill in U.S. history.

BP began pumping mud into the well on Wednesday afternoon but stopped late Wednesday night to assess how the effort was working and to bring in more materials. Pumping resumed Thursday evening.

The company also may shoot material into a crippled piece of equipment atop the well to keep the mud from escaping.

It could be late Friday or the weekend before the company knows if it has cut off the oil that has been flowing for five weeks.

(Copyright 2010 by The Associated Press. All Rights Reserved.)

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