BP told US congressional investigators that pressure tests on a drill pipe showed a fundamental mistake hours before the deadly explosion that caused the Gulf of Mexico oil leak, a memo released by two congressmen reveals.

The memo by Representatives Henry Waxman and Bart Stupak, who were briefed by BP about the progress of its internal investigation, also said problems were found in equipment meant to provide fail-safe protection against a blowout, reports Reuters.

A BP investigator indicated to the lawmakers that the fundamental mistake may have been made because heavy pressure on the drill line of 1,400lb per square inch (98kg per square centimetre) was an “indicator of a very large abnormality”.

The memo did not indicate who made decisions after the problem was found. BP and Transocean, the owner and operator of the rig, had supervisors on the rig when it exploded.

Meanwhile, BP has been preparing to use a method known as top kill, which involves injecting heavy drilling fluid and cement near the top of the well, in the latest attempt to plug the leak, which BP says is gushing 5,000 barrels per day even though some scientists estimate it could be up to 20 times more.

BP also aims to remove a damaged part from the ruptured well and put in place a tube that would capture most of the oil and gas flowing into the sea.