Federal regulators warned offshore rig operators more than a decade ago that they needed to install backup systems to control the giant undersea valves known as blowout preventers, used to cut off the flow of oil from a well in an emergency. The warnings were repeated in 2004 and 2009. Yet the Minerals Management Service, the Interior Department agency charged both with regulating the oil industry and collecting royalties from it, never took steps to address the issue comprehensively, relying instead on industry assurances that it was on top of the problem, a review of documents shows. In the intervening years, numerous blowout preventers and their control systems have failed, though none as catastrophically as those on the well the Deepwater Horizon drilling rig was preparing when it blew up on April 20, leaving tens of thousands of gallons of oil a day spewing into the Gulf of Mexico. Agency records show that from 2001 to 2007, there were 1,443 serious drilling accidents in offshore operations, leading to 41 deaths, 302 injuries and 356 oil spills. Yet the federal agency continues to allow the industry largely to police itself. Critics say that, then and now, the minerals service has been crippled by this dependence on industry and by a climate of regulatory indulgence.