(CNN) -- Four Transocean workers have refused to appear before the Coast Guard-Interior Department inquiry into the sinking of the Deepwater Horizon drill rig, forcing the panel to cancel a Wednesday hearing into the worst oil spill in U.S. history.
All four worked on the doomed rig's blowout preventer, the massive fail-safe device that has failed to cut off the undersea gusher at the heart of the disaster. They "declined to voluntarily appear" before the joint Coast Guard-Interior Department board investigating the sinking, the panel said in a statement canceling the hearing.
Coast Guard Petty Officer Elizabeth Bordelon said the men are from the Houston, Texas, area, as are many of the BP and Transocean employees slated to testify in upcoming hearings, and cited problems with travel arrangements for their cancellation. The rest of the week's proceedings are expected to go on as planned, and the men's testimony has been rescheduled for August in Houston, where the board plans its next round of hearings, she said.
"We don't anticipate any problems for the rest of the week or the future set of hearings," said Bordelon, a spokeswoman for the investigative board.
Transocean owned the Deepwater Horizon, which sank two days after an April 20 explosion off Louisiana that left 11 crew members dead. It leased the platform to BP, the owner of the ruptured well, and BP, Transocean and cementing contractor Halliburton all have blamed each other for the disaster.
The investigative panel is charged with finding the cause of the disaster "to the fullest extent possible" and could refer any findings of misconduct to federal prosecutors. The panel has a January 2011 deadline for completing its report, Bordelon said.
"The investigation is ongoing," she said. "They're working around the clock. They're working diligently to get this report compiled and make sure it's as accurate and complete as can be."
Monday was the first day of the panel's third round of hearings, and it was marked by contentious exchanges between board members and lawyers for several figures in the investigation.
Attorneys for the captain and the top drilling manager aboard the rig criticized the board for declaring their clients "parties in interest" to the investigation after they testified in May. The designation indicates their actions are under scrutiny as part of the investigation, and it would have allowed their attorneys to cross-examine witnesses and review documents on their behalf had it been granted at the time they testified.
In addition, an attorney for Transocean complained that the investigative board was trying to intimidate witness Stephen Bertone, the doomed rig's chief engineer. The lawyers also objected sharply to efforts to question Bertone about a statement he gave in the hours after the explosion that painted the rig's captain, Curt Kutcha, in an unflattering light.
Bertone, meanwhile, requested and received party-in-interest status after Monday's hearing, Bordelon said.
Tuesday, a BP official testified that a control unit on the Deepwater Horizon's blowout preventer was leaking hydraulic fluid before the explosion that sank the rig. Ronald Sepulvado said there were "off and on" problems with valves that directed hydraulic fluid inside the mechanism, and the rig continued to operate after he reported the problem to his superiors.
Federal regulations require that drilling work halt if a control "pod" is inoperable. Sepulvado said the unit was still functional, but his team leader should have reported the issue to the Minerals Management Service, the recently reorganized Interior Department agency that regulated offshore drilling.
A second BP official aboard the rig invoked his Fifth Amendment right against self-incrimination, the second time he has done so in two rounds of hearings.