After several failed attempts at stopping the oil leak in the Gulf of Mexico, BP started the top kill procedure on Wednesday in hopes of sealing the well shut permanently with cement. But BP PLC's Chief Executive Tony Hayward said that this procedure only had "a 60 to 70 percent chance of long-term success."

The top kill procedure is a process where heavy fluid under pressure is pumped into the broken blowout preventer. The idea is that the heavy fluid should counteract the pressure of the oil and suppress it back into the well where cement will be injected to seal it. 

In the early stages of the top kill procedure, BP noticed that the heavy fluid, or kill mud, was flowing up to the surface and out to sea rather than pressing down into the well. Their resolution was the junk shot procedure, which is a maneuver BP officials have compared to "plugging up a toilet" with shredded rubber tires, golf balls and pieces of rope that are being pumped into the well. BP has reported that there were some indications that the junk shot was helping.

Hayward mentioned that it could take as long as 48 hours to conclude whether the top kill procedure is a success or not, but BP engineers have stated that oil stop flowing out of the well they pump the kill mud into it. Yet, the kill mud hasn't overwhelmed the oil completely and the well is certainly not ready for the cement seal.

BP said the total cost to respond to the Gulf of Mexico oil spill so far is $930 million. This figure includes federal costs and claims paid. 

President Barack Obama took full responsibility for what has become the worst oil spill in U.S. history, and went to Fourchon Beach in Louisiana to supervise the spill effort. 

Thad Allen, Coast Guard admiral in charge of the spill response, said he was "very encouraged" that BP was able to weigh the oil down, but "the real question is, can we sustain it."

"That will be the critical issue going though the next 12 to 18 hours," said Allen.