MarketWatch reports, the lead U.S. government official in charge of the Deepwater Horizon oil spill said the final drilling to kill the leaky Macondo well began on Wednesday morning. National Incident Commander Thad Allen said the Development Driller III is drilling a relief well that should intercept BP PLC''s Macondo well by Thursday. After additional testing, drilling mud and cement will be pumped into the bottom of the well to seal it off from the oil and gas reservoir below it. The well may be sealed within four days, he said. The spill began after an explosion on the Deepwater Horizon well on April 20 killed 11 workers and caused nearly 5 million barrels of oil to spill into the Gulf of Mexico. The well has been shut in since July 15.