The credibility of the investigation into the Gulf oil spill is being undermined because representatives of companies that made or maintained a key piece of evidence — the blowout preventer — have had too much access to it as it is being analyzed, a federal board says.

The U.S. Chemical Safety Board, which is being allowed to monitor the analysis, demanded in a letter Thursday to the head of the Bureau of Ocean Energy Management, Regulation and Enforcement that testing stop and not resume until Transocean and Cameron officials are removed from any hands-on role in the examination of the 300-ton device.

An employee of Transocean — the owner of the drilling rig that exploded in the Gulf — has been removed as a consultant for the Norwegian firm conducting the testing, but the ocean energy bureau says that otherwise the companies have provided their expertise appropriately. The board claims conflicts still exist.

The board, like the companies and other parties involved, has been granted limited access to the testing, but it says its representatives have been shut out of tests that have included multiple representatives of Transocean and Cameron International, which made the blowout preventer. The board also wants the firm leading the testing, Det Norske Veritas, terminated or at least supervised by a neutral third party. It’s also demanding photo and video evidence of work conducted while its representatives were shut out of testing.

“Given the well-publicized history of improper relationships between the former Minerals Management Service and members of the oil industry, one would have expected that extraordinary care would be taken to conduct the BOP testing above reproach,” safety board chairperson Rafael Moure-Eraso said in the letter. “One would have expected an independent, second set of eyes like the CSB to be welcomed. Regrettably this has not been the case.”

The MMS was renamed the Bureau of Ocean Energy Management, Regulation and Enforcement after two scathing reports by a federal inspector general. The reports highlighted drug use and sex among agency employees and oil and gas industry executives, and said drilling regulators accepted gifts and trips from oil and gas companies and even negotiated to go work for the industry while overseeing it.

A Joint Investigation Team that includes bureau personnel is leading the blowout preventer probe along with the U.S. Coast Guard. The safety board is an independent federal agency that investigates serious chemical accidents.

Following the April 20 rig explosion in the Gulf of Mexico, the blowout preventer used with BP’s well failed to do its job: stopping the flow of oil to the sea. Eleven workers were killed in the blast, and some 200 million gallons of oil were released by BP’s undersea well, according to government estimates that BP disputes. The device was raised from the seafloor on Sept. 4, and the testing process began Nov. 16 at a NASA facility in New Orleans. Technicians have largely been disassembling the blowout preventer and have so far made no determination about why it didn’t work, according to a person briefed on the progress who spoke on condition of anonymity because of the sensitivity of the investigation.

Blowout preventers sit at the wellhead of exploratory wells and are supposed to lock in place to prevent a spill in the case of an explosion. They can snuff a blowout by squeezing rubber seals tightly around the pipes with up to 1 million pounds of force. If the seals fail, the blowout preventer deploys a last line of defense: a set of rams that can slice right through the pipes and cap the blowout.

In a letter sent earlier this month to Michael Bromwich, director of the ocean energy bureau, the safety board included a picture of a Transocean subsea supervisor, Owen McWhorter, removing the upper pipe ram of the blowout preventer on Dec. 10. A spokeswoman for Bromwich’s agency, Melissa Schwartz, said Thursday that DNV, the firm handling the testing, allowed McWhorter to do that work unbeknownst to federal investigators. She said McWhorter was removed from the process within days.

Schwartz said the companies have been permitted to provide technical expertise through an agreement between the parties for the sole purpose of answering any technical questions that DNV personnel performing the examination may have. Representatives from these companies are observers only and are not involved in the examination, she said.

Transocean said in a short statement e-mailed to the AP that the Chemical Safety Board’s “accusations are totally unfounded.” It didn’t elaborate.

A spokesman for Det Norske Veritas, Blaine Collins, said in an e-mail to AP that he could not comment, referring questions to Bromwich’s agency.

Aside from the board, Cameron and Transocean, others allowed to monitor the testing are BP, the Department of Justice and an expert representing the plaintiffs in the multi-district oil spill lawsuits in New Orleans.