Federal investigators became visibly annoyed Thursday as BP executives were unwilling or unable to provide a clear picture of the company's heirarchy or say who was in charge of the rig leased by the oil giant the day it blew up.

"Everybody in charge, nobody in charge," U.S. Coast Guard Capt. Hung Nguyen, co-chairman of the investigative board, said in frustration after repeatedly questioning BP senior vice president Kent Wells about who oversaw safety on the Deepwater Horizon, and the company as a whole.

"I disagree," Wells responded, explaining that BP aimed to instill a company-wide safety culture that ensured every employee felt responsible.

Other high-level BP employees testified during a fourth day of hearings in Houston, revealing a major managerial restructuring — including the replacement of a top rig official just four days before the Deepwater Horizon exploded — led to confusion in the days before the rig exploded April 20, killing 11 people.

The testimony also revealed that key work procedures on the rig may have been changed in the days and even hours before the blast, possibly without receiving the necessary federal approval. Furthermore, David Sims, BP's drilling and completions operations manager, testified that a move from a paper to electronic system for filing major decision changes caused further confusion and was a "painful" process.

Attorneys and the joint panel of the U.S. Coast Guard and the Bureau of Ocean Energy Management, Regulation and Enforcement spent the bulk of the hearings seeking basic information about BP's personnel structure. Yet those details remained out of reach, with Nguyen and other board members repeatedly voicing their frustration.

Lawyers from BP, rig owner Transocean, victims and others have also been quick to object to questions. Those testifying often gave vague answers or simply said: "I don't recall."

Wells, who has been key to BP's efforts to plug the largest oil spill in U.S. history, told the panel repeatedly he had not asked questions or interested himself in what led to, or possibly triggered, the April 20 explosion that killed 11 people. He said this was to ensure he would be fully focused on the response.

Asked repeatedly if there was any one person in the company responsible to enforce its so-called "golden rules" of safety, he said "our culture is meant that everyone is supposed to feel responsible." No matter how the question was posed, the answer remained largely the same.

Wells was also asked about the drilling of two relief wells and whether they would serve as the final plug. He skirted the question and said there are "multiple options" to stop the oil and the relief wells are "the ultimate backup if everything else fails."

Retired Coast Guard Adm. Thad Allen, the government's point man on spill response, has insisted the relief wells would be the final solution.

After Wells testified, a BP spokesman said the company and Allen "remain aligned regarding relief well plans."

Later, Sims — who was on the rig at the time of the explosion — took the stand, insisting he could not remember conversations he had after the explosion with key managers and decision makers, though he admitted there was a lot of speculation in the hours and days after the incident over what may have been the trigger.

He said he was aware of a debate among the company's top rig engineers and drillers over whether to go with a Halliburton plan for cementing the well or proceed with the original BP lineup. Halliburton says it had warned, based on computer models, that BP's plan could cause severe gas flow problems. In the end, the BP decision makers chose to go with their own plan, ignoring Halliburton's warnings.

Sims said he did not see the e-mails talking about the gas flow problems until days after the explosion.

E-mails revealed in the hearings also indicated last-minute changes were made to a plan for conducting a crucial test before cementing the well, possibly without getting the required approval from what was then the Mineral Management Service.