<P> At 09: 15 CDT on August 4, with Adm. Allen's approval, BP began pumping cement from the top, sealing that part of the flow channel permanently . </P> <P> On August 4, Allen said the static kill was working . Two weeks later, though, Allen said it was uncertain when the well could be declared completely sealed . The bottom kill had yet to take place, and the relief well had been delayed by storms . Even when the relief well was ready, he said, BP had to make sure pressure would not build up again . </P> <P> On September 10, Allen said the bottom kill could start sooner than expected because a "locking sleeve" could be used on top of the well to prevent excessive pressure from causing problems . BP said the relief well was about 50 feet (15 m) from the intersection, and finishing the boring would take four more days . On September 16, the relief well reached its destination and pumping of cement to seal the well began . Officials said on September 18 that the cement pumped in from the base of the well had completed the sealing of the well . On September 19, after pressure testing, Allen declared the well "officially dead". </P> <P> In mid-May 2010, United States Secretary of Energy Steven Chu assembled a team of nuclear physicists, including hydrogen bomb designer Richard Garwin and Sandia National Laboratories director Tom Hunter . On May 24, BP ruled out conventional explosives, saying that if blasts failed to clog the well, "We would have denied ourselves all other options ." </P>

When did the bp oil spill stop leaking