“I don’t want to dwell on ‘coulda, shoulda, woulda,’” he said. “There’s nothing I can really point to that we shouldn’t have done based on what we knew at the time.”

Image Steven Chu, a Nobel Prize winner gaining new expertise. Credit... Drew Angerer/Associated Press

Dr. Chu, former director of the Lawrence Berkeley National Laboratory, said: “When things are unfolding, there are lots of possibilities and false starts. If you knew the right path at the beginning, could it have been done a little faster? Probably.”

He said he became involved in the early days of the spill response as a scientific diagnostician  he used the term “kibbitzer”  and not as a decision maker. Early on, he suggested the use of high-energy gamma rays to see inside the wrecked blowout preventer, providing data that helped guide later steps at controlling the oil flow.

His active involvement in decision making in BP’s Houston control room began in early May after he raised a number of technical points about the blowout in a meeting with President Obama and advisers in the White House Situation Room. “You better get down there,” the president said, according to an aide who was in the meeting.

Dr. Chu has since made six extended trips to the gulf, conferring with his own scientific team as well as top BP engineers and executives in the company’s command center. He spends several hours a day on the problem, on the telephone or in meetings with his advisers and with BP officials. He is not formally part of the chain of command in the spill response, but carries the authority of Mr. Obama as well as his own considerable intellectual heft.

His role gradually deepened as he assembled a team of scientists from the Department of Energy laboratories, universities and other government agencies. By late May, his confidence had grown and he was giving orders to BP officials, including his demand to stop the top kill effort even though some BP engineers believed it could still succeed.