Mr. Newman also acknowledged that he knew Transocean had problems assuring safety even before the Deepwater Horizon accident when four workers were killed in rig accidents in 2009. The company temporarily suspended operations and underwent a review of its safety practices. “I knew that as a result of the incidents we were experiencing we needed to do something,” he said.

Mr. Newman testified for more than five hours with a steady voice. But at the end of his testimony, when asked how often he thought of the men who lost their lives in the accident, he choked up and fought back tears. “Every day,” he answered. “Because I ask myself if there isn’t something more I could have done.”

Another contractor-defendant, Halliburton, which mixed the cement for the well, has also faced some embarrassing questions at the trial in recent days. Thomas Roth, a senior Halliburton executive who was in charge of cementing operations at the time of the spill, acknowledged that because of the well design and other factors, “the cement placement was going to be a job that would have a low probability of success.”

Timothy Quirk, a Halliburton laboratory manager, testified on Tuesday that he conducted stability tests on cement samples from a similar blend that had been used in the Macondo well after the accident. Under instructions from a colleague, he said he did not prepare a laboratory work sheet. “It was a little unusual,” he conceded. He then acknowledged that he had thrown out his notes. Subsequent investigations have shown that the cement was not stable.

The plaintiffs have accused Halliburton of conducting undocumented cement tests and then hiding the results. BP has accused Halliburton of destroying evidence of its cement testing.

“To shift responsibility to Transocean and Halliburton is good for BP,” said Edward F. Sherman, a law professor at Tulane University. “They would like to argue that the primary actors were Halliburton and Transocean employees and BP was not responsible for their failures and, therefore, BP could not be grossly negligent.”