Only seven of the 126 onboard the Deepwater Horizon rig were BP employees when it was engulfed in flames, he stressed.

However, Transocean's Newman said the blowout preventers "were clearly not the root cause of the explosion" and "it is inappropriate to focus any causation discussions exclusively on the blowout preventers." He said they might have been damaged by debris made of cement and steel casing material blown upward because of other failures.

"The one thing we know with certainty is that on the evening of April 20 there was a sudden, catastrophic failure of the cement, the casing, or both," Newman said. "Therein lies the root cause of this occurrence. Without a disastrous failure of one of those elements, the explosion could not have occurred."

Newman also emphasized BP's role.

"Offshore oil and gas production projects begin and end with the operator, in this case BP," said Newman. He said it was BP that prepared the drilling plan and BP that gave the go-ahead to fill the well pipe with sea water before a final cement cap was installed, reducing the downward pressure.

Halliburton's Tim Probert also tried to deflect blame, saying " we understand that the drilling contractor ... proceeded to displace the riser with seawater prior to the planned placement of the final cement plug." That plug was designed to keep the oil and gas in the well, a final step before pulling the drilling rig away from the well. Probert said that the final cement plug was never set.

Probert insisted that the company's work was done "in accordance with the requirements" set out by BP and followed accepted industry practices.

However, lawmakers made clear that they didn't like the finger pointing.

"I hear one message. And the message is, don't blame me," said Republican Senator John Barrasso. "Well, shifting this blame does not get us very far."

Tuesday's hearing is the first public questioning by members of Congress of executives of BP, Swiss-based Transocean and Halliburton to probe the rig disaster.

It has been nearly three weeks since the explosion and every day 5, 000 barrels of oil is still leaking into the gulf. Currently the oil has reached the shores of Louisiana, impacting the livelihoods of millions in the Gulf Coast states and threatening more.