Mr. Holloway returned to work, and he and the other floorhands got busy cleaning the drilling floor. They avoided the drill shack, though. Lately, there had been too much stress there.

Mr. Holloway could tell when the BP “company men” got on Mr. Revette’s nerves: he would rub his head a certain way. This had happened a lot on the Macondo job. The Horizon might have been Transocean’s rig, but it was BP’s well, and it was obvious that the guys in the shack felt that the BP men were breathing down their necks. “You could just tell,” Mr. Holloway said.

BP has denied pressuring the Horizon’s crew to cut corners, but its plans for completing the well kept changing, often in ways that saved time but increased risk. “It’s a new deal every time we get up,” Jason Anderson, a 35-year-old toolpusher, complained to his father.

By early evening, there was one crucial test remaining before the Horizon could plug the Macondo and move on. To make sure the well was not leaking, the crew would withdraw heavy mud from it and replace it with lighter seawater. Then they would shut in the well to see if pressure built up inside. If it did, that could mean hydrocarbons — oil and gas — were seeping into the well.

In effect, they were daring the well to blow out. Designing and interpreting this test — a “negative pressure” test — requires a blend of art and engineering expertise. There are no industry standards or government rules. Designing the test was BP’s job, yet the oil company’s instructions, e-mailed to the rig that morning, were all of 24 words long.

It fell to the BP company men and the drilling crew to work out the details, but it did not go well. There was strong disagreement over the test results. Pressure had built up, exactly what they did not want, and Mr. Wheeler, the team’s toolpusher, was worried. A BP company man said he thought the test went fine. Other rig managers, including Mr. Anderson, joined the discussion, and they debated whether to repeat the test.

By 8 p.m., after redoing the test, they all agreed that the Macondo was stable. In a few hours, the drilling crew’s 21-day hitch would be done. They were working unusually fast. In seven years on the Horizon, Joseph Keith had never seen so much activity while sealing a well, and it made him uncomfortable. His job included monitoring gauges that detect blowouts. But all the jobs going on at once — transferring mud to a supply vessel, cleaning mud pits, repairing a pump — could throw off his instruments. Mr. Keith did not tell anyone that he was worried about his ability to monitor the well. “I guess I just didn’t think of it at the time,” he later testified.