WASHINGTON (Reuters) - BP Plc told congressional investigators on Tuesday 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 showed.

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 blow out.

A BP investigator indicated to the lawmakers that the fundamental mistake may have been made because heavy pressure on the drill line of 1,400 pounds per square inch (98 kg per square cm) 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.

About two hours before the explosion that killed 11 workers and caused the leak that is still gushing oil, the rig team was satisfied a test on another line was successful, the BP investigator said.

BP would not comment on the memo.

A Transocean spokesman blamed BP for the disaster, saying, "A well is constructed and completed the same way a house is built -- at the direction of the owner and the architect. And in this case, that's BP."

The congressional memo said BP data showed there were numerous problems with production equipment. Nearly five hours before the explosion, an unexpected loss of fluid was observed in the well's riser pipe. That suggests "there were leaks in the annual preventer," a rubber gasket in the blowout preventer, the memo said.

Waxman and Stupak chair the House Energy and Commerce Committee's investigative subcommittee, which has reviewed over 105,000 pages of internal documents from BP, rig owner Transocean Ltd, Halliburton Co, and blowout preventer manufacturer Cameron.

The memo said the BP investigation has also raised concerns about the maintenance history, modification, and inspection of the blowout preventer.

Officials from BP and Transocean are scheduled to testify before congressional panels on Thursday.

According to BP, there also were three warning signs of problems with unwanted flow in the well starting 51 minutes before the explosion. About 18 minutes before the explosion abnormal pressure leaks of the fluids known as mud meant to keep oil and gas capped were observed and the pump was shut down.

"The data suggests that the crew may have attempted mechanical interventions at that point to control the pressure, but soon after, the flow out and pressure increased dramatically and the explosion took place," the memo said.

(Reporting by Timothy Gardner, editing by Philip Barbara and Bill Trott)