WASHINGTON (AFP) – Multiple warnings signs preceded an explosion that tore through a BP-leased oil rig, killing 11 and precipitating a massive oil leak in the Gulf of Mexico, US lawmakers have said.

Citing a preliminary BP inquiry into the disaster, Representatives Henry Waxman and Bart Stupak said there were three worrying flow indicators from the well in the hour before the accident alone.

But a series of problems were also detected in the 24 hours before the blast, including unexpected pressure rises and fluid leaks, flaws in cementing done to the well, and deviations by rig workers from a drilling plan.

The warning signs kept coming right up until just minutes before the Deepwater Horizon rig exploded on April 22, damaging a well pipe that has since leaked tens of thousands of barrels of oil into the ocean.

Around 51 minutes before the accident, "more fluid began flowing out of the well than was being pumped in," the two lawmakers who chair the Energy and Commerce Committee said in a memo.

Then 41 minutes before the explosion, despite being shut down for a test, "the well continued to flow instead of stopping, and drill pipe pressure also unexpectedly increased."

And just 18 minutes before the explosion "abnormal pressures and mud returns were observed and the pump was abruptly shut down," the memo added.

Data collected by the investigation suggested the oil rig workers may have tried to mechanically control the pressure, but instead it "increased dramatically," leading to the destruction of the rig some 50 miles (80 kilometers) off the Louisiana coast.

The investigation, which the lawmakers cautioned was based on the incomplete information currently available, said other warning signs emerged around five hours before the blast, when an unexpected loss of fluid was observed in the riser pipe.

That could have indicated leaks in the blowout preventer, a device intended to shut off the oil well in case of an accident, which subsequently failed to operate when workers tried to activate it.

Two hours before the explosion, 15 barrels of liquid flowed into the system as negative pressure testing began, instead of the usual five barrels.

A volume of fluid also came out when the "kill line" was opened for testing, the memo said, prompting workers to close the line and discuss what action they should take.

Pressure began rising, so the workers decided to bleed the line, which a BP investigator said may have been a "fundamental mistake" because the pressure build-up was an "indicator of a very large abnormality."

The investigation also found that "cement work that was supposed to hold back hydrocarbons failed," and that a "float collar," a device used during the cementing process, "required nine attempts with higher than usual pressure to function properly."

The investigation identified at least four problems with the blowout preventer and, the memo said, "key questions exist about whether proper procedures were followed for critical activities throughout the day."

Over a month after the disaster, BP has failed to cap the leak and made limited progress in containing the flow of oil, which has begun washing up on beaches and fragile marshlands in Louisiana.