Sept. 20 (Bloomberg) -- U.S. officials said BP Plc killed its Macondo well in the Gulf of Mexico after creating another cement seal, plugging the source of the largest offshore oil spill in U.S. history. "The Macondo 252 well is effectively dead," said National Incident Commander Thad Allen in a statement yesterday. BP completed its last pressure test on the plugs at 5:54 a.m. local time before declaring the well sealed, according to the statement. Susan Li reports on Bloomberg Television's "First Up."