Federal investigators are warning that the participation of companies involved in the Gulf Coast oil disaster is compromising a government probe. The chair of the Chemical Safety and Hazard Investigation Board, Rafael Moure-Eraso, criticized a decision allowing firms to take part in examining the blowout preventer that failed to stop the oil rig’s explosion. In a letter of protest issued last week, Moure-Eraso writes that the companies’ involvement has “seriously undermined the credibility of the testing” and “jeopardizes the public’s trust in the examination process.”