Oil executives are being questioned by a US senate committee three weeks after an oil rig explosion and fire killed 11 men and triggered a disastrous oil leak in the Gulf of Mexico.

Al Jazeera's Rob Reynolds reports from Washington DC where hearings were held on Tuesday in which senior executives from BP, Transocean and Halliburton were asked to account for the "cascade of failures" at the Deepwater Horizon drilling platform prior to the catastrophic rupture in the wellhead.

Tuesday's back-to-back hearings were the first congressional inquiry into the April 20 incident, which has since grown a massive environmental and economic disaster.

BP was the exploratory well's owner and overall operator, Transocean the rig's owner and Halliburton a subcontractor that was encasing the well pipe in cement before plugging it in anticipation of future production.

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The three companies' executives took turns blaming each other when asked about lax safety and unpreparedness.

Each company defended its own operations and raised questions about its partners in the project.

The corporate finger-pointing prompted an admonishment from Lisa Murkowski, the Republican senator of oil-rich Alaska, who said "there will be no excuse" if operators are found to have violated the law.

"This accident has reminded us of a cold reality, that the production of energy will never be without risk or environmental consequence," she said.

A BP spokesman said an oil containment box known as a "top hat" was being taken to the leaking oil well and undersea robots would position it over the gusher by Thursday.

The new device is much smaller than one that failed during the weekend and engineers hope it will cap the leak and allow the oil to be brought up to waiting tankers on the surface.

Ramifications from the environmental crisis spilled over into landmark climate change and energy legislation that is coming out Wednesday.

The bill proposes letting coastal states veto drilling projects off the shores of neighbouring states if they can show the potential for harm.