Executives of three oil and oil service companies involved in the massive Gulf of Mexico oil spill all blame each other at a hearing Tuesday at the U.S. Senate Energy and Natural Resources Committee.

Lamar Mckay (L), President and Chairman of BP America, Inc., Steven Newman (C), President and Chief Executive Officer of Transocean Limited, and Tim Probert, President of Global Business Lines and Chief Health, Safety and Environment Officer Halliburton, attend a hearing on offshore oil drilling before the U.S. Senate energy and natural resources committee at the Capitol Hills in Washington D.C., capital of the United States, May 11, 2010. The hearing to review issues related to deepwater offshore exploration for petroleum and the accident in the Gulf of Mexico involving the offshore oil rig Deepwater Horizon. [Zhang Jun/Xinhua]

Lamar McKay, president of BP America, Steven Newman, chief executive of Transocean and Tim Probert, a senior executive of Halliburton, testified before the committee. Their companies face intense political pressure in the aftermath of an explosion that sank Transocean's Deepwater Horizon rig as it was finishing a well for BP. The accident killed 11 people and oil is still flowing unchecked from the ruptured well.

Halliburton joins BP and Transocean because it provided a variety of services on the rig and was involved in cementing on the well to stabilize its walls.

While acknowledging that it was too early to make a final determination of the cause of the April 20 blowout that started the spill or why the oil was not contained, the companies stressed one another's failures.

BP's McKay focused on a critical safety device, the 450-ton blowout protector owned by rig operator Transocean, that was supposed to shut off oil flow on the ocean floor in the event of a well blowout but "failed to operate."

"The systems are intended to fail-close and be fail-safe," McKay said. "Sadly and for reasons we do not yet understand, in this case, they were not. Transocean's blowout preventer failed to operate."