Wall Street Journal- Aug. 26, 2010
"BP Casts Blame for Failure of Device"
"Undocumented changes to the blowout preventer that should have shut down BP PLC's runaway Gulf well slowed efforts to stop the flow of oil, a BP official [Thierens] testified Wednesday.
After spending days trying to use underwater robots to trigger the blowout preventer, ...... workers discovered that it had been modified........
"The controls that the robots were trying to use that were supposed to close the valves were actually connected to a testing device that couldn't shut off the well, he said,"
......."Even after they learned of the modifications, workers still couldn't make the blowout preventer work.
"BP has said that Transocean bears at least partial responsibility .....because of the equipment's failure, and the companies have been casting blame on each other."
"...Transocean said that the blowout preventer had passed numerous tests.......and the modifications had no impact on efforts to shut down the well afterward."

I am quoting excerpts, because reports can be withdrawn. The report in the Telegraph linked by Teribus can't be accessed now.