NEW ORLEANS – A federal board allowed to monitor testing of a key piece of Gulf oil spill evidence — the blowout preventer — demanded Thursday that the analysis stop, saying representatives of the companies that made and maintained the 300-ton device have been getting preferential and sometimes hands-on access to it.

The U.S. Chemical Safety Board said in a letter to the head of the Bureau of Ocean Energy Management, Regulation and Enforcement that having the companies involved hands-on in the forensic analysis that began more than a month ago undermines the investigation's credibility.

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 wants the testing stopped and for it not to resume until Transocean and Cameron officials are removed from any hands-on role in the examination.

It 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.