Executives of the three companies involved in one of the worst spills in U.S. history acknowledged in testimony before a Senate panel that investigators have yet to pinpoint exactly what caused the disaster, but they spent little time before trying to shift responsibility for the crisis to each other. Transocean CEO Steven Newman...said BP, which leased the rig, bore responsibility...But BP America Chairman Lamar McKay pointed the finger at Transocean, which owned the failed 450-ton blowout preventer that was supposed to shut off oil flow on the ocean floor...Halliburton executive Tim Probert, in turn, said his company completed its operations 20 hours before the rig went up in flames...The massive Gulf Coast spill has prompted the Obama administration to rethink the agency charged with oversight of offshore drilling [the Minerals Management Service]...