Emergency shut-off valves that could have prevented the Gulf of Mexico oil spill were plagued by leaking hydraulic fluid, malfunctioning parts and poor maintenance, Congressional investigators said Wednesday.

The problems with the blowout preventer on the Deepwater Horizon drilling rig could explain why it failed April 20, allowing a spout of oil and gas to explode and kill 11 workers, according to documents released by the House energy committee's Oversight and Investigations Subcommittee.

 "The safety of its entire operations rested on the performance of a leaking, modified, defective blowout preventer," said Rep. Bart Stupak, D-Mich., chairman of the subcommittee.

 House investigators also revealed that the well had failed at least two pressure tests in the hours before it blew, raising questions about whether cement recently poured into the well could hold back the high-pressure oil and gas.