Published: May 11, 2010 5:37 PM EDT

WASHINGTON  - The CEO of the company that owned the oil drilling rig that exploded in the Gulf of Mexico says the accident was caused by a failure of its cementing, casing or perhaps both.

Transocean Ltd. CEO Steven Newman said at a Senate hearing Tuesday that the April 20 explosion of the Deepwater Horizon rig was unusual because it happened after construction of the well was essentially complete.

He said that the explosion that killed 11 workers could not have happened unless the cement, well casing -- or both of those elements -- failed. The rig sank and has been gushing oil into the Gulf. Newman dismissed suggestions that a blowout preventer owned by Transocean may have been a cause.

Worker: Firefighting at Gulf rig was uncoordinated

A Coast Guard worker says he wasn't aware of anyone coordinating efforts to put out a fire on a drilling rig that exploded in the Gulf of Mexico last month. Kevin Robb, a civilian Coast Guard worker who helped coordinate search and rescue efforts after the blast April 20, made the remarks at a hearing Monday in suburban New Orleans.

When asked whether uncoordinated firefighting efforts could lead to the sinking of a disabled vessel, Robb said that was "exactly correct." He said putting out fires out on the water requires a great deal of training and coordination.

Robb said a platform owner may be responsible for enlisting somebody to coordinate firefighting efforts. But he added that he was unsure of who was responsible for coordinating those efforts.