In this image made from video released by British Petroleum (BP PLC), oil can be seen pouring out of the blowout preventer, left, as robot submarines work near it early Wednesday June 2, 2010. (AP Photo/BP PLC)
The U.S. Chemical Safety Board said in a letter Thursday that having the companies involved hands-on in the forensic analysis that began more than a month ago undermines the investigation's credibility.
However, while Cameron and Transocean officials are among a small army of representatives allowed to watch and consult, they are not involved in the actual testing. That's being done by a Norwegian firm.
The board said it also wants that firm terminated or at least supervised by a neutral third-party.
The safety board investigates serious chemical accidents. It had previously agreed to the current test protocols.