By Siobhan Hughes Of DOW JONES NEWSWIRES WASHINGTON (Dow Jones)--The U.S. Chemical Safety and Hazard Investigation Board said it has opened an investigation into the causes of the April 20 explosion of a drilling rig leased by BP PLC (BP, BP.LN). The board "intends to proceed with an investigation of the root causes of the accidental chemical release that destroyed the Deepwater Horizon rig and took the lives of 11 workers," John Bresland, the board's chairman, wrote in a June 18 letter to House Energy and Commerce Committee Chairman Henry Waxman (D, Calif.) He wrote that the board intends to focus on events leading up to and including the explosion. He said that an examination of the response to the disaster and the impact of a subsequent oil spill "is beyond" the board's resources and abilities. The explosion and subsequent damage to a mile-deep BP oil well has sent oil gushing into the gulf. The disaster has become the worst offshore oil spill in U.S. history, according to U.S. records. -By Siobhan Hughes, Dow Jones Newswires; (202) 862-6654; siobhan.hughes@dowjones.com (END) Dow Jones Newswires June 21, 2010 17:05 ET (21:05 GMT)