Thierens says as attempts were being made to shut the well with the blowout preventer it became clear rig-owner Transocean had made changes to the device's locking mechanism.
He says it took between 12 hours and 24 hours to get the right drawings of the changes. In the end, the device failed.
Meanwhile, the chairmen of the presidential panel investigating the Gulf oil spill are expressing disappointment that the Obama administration didn't consult with senior U.S. environmental officials before announcing plans to expand offshore drilling before the accident.
The exchange during a hearing Wednesday suggests that a focus of the federal investigation will be the degree to which federal scientists were consulted in oil and gas decisions.
Both the chairwoman of the Council on Environmental Quality and the head of the National Oceanic and Atmospheric Administration told the panel they were not directly involved in Obama's March announcement to expand offshore drilling.
- The Gulf of Mexico oil spill has revealed a previously unknown type of oil-eating bacteria, which is suddenly flourishing. Scientists discovered the new microbe while studying the underwater dispersion of millions of gallons of oil spilled into the Gulf following the explosion of BP's Deepwater Horizon drilling rig.
- Scientists also had been concerned that oil-eating activity by microbes would consume large amounts of oxygen in the water, creating a "dead zone" dangerous to other life. But the new study found that oxygen saturation outside the oil plume was 67 percent, while within the plume it was 59 percent.
- The cleanup of history's worst peacetime oil spill is generating thousands of tons of oil-soaked debris that is ending up in local landfills, some of which were already dealing with environmental concerns.
- The soft, absorbent boom that has played the biggest role in containing the spill alone would measure more than twice the length of California's coastline, or about 2,000 miles. More than 50,000 tons of boom and oily debris have made their way to landfills or incinerators, according to federal officials, representing about 7 percent of the daily volume going to nine area landfills.
The Associated Press contributed to this report.