Washington (CNN) -- The public may have seen the last video images of the failed blowout preventer from the BP oil well disaster, as authorities expressed possible security concerns.

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The remote-controlled submarines are being used by BP in an attempt to activate the well's blowout preventer to choke the flow of oil.

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BP, which had been leasing the rig from Transocean, says the safety device that was supposed to cut oil flow on the ocean floor in the event of a well blowout did not perform its function.

FORBES

Transocean ( RIG - news - people ), says the safety device that was supposed to cut oil flow on the ocean floor in the event of a well blowout did not perform its function.

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At that depth, a key piece of equipment is the blowout preventer, which weighs up to 300 tons and caps the well, holding back ultrapressurized oil deep in the Earth.

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As a prelude to permanently shutting down the well, crews are trying to withdraw drill pipe hanging below the bottom of the blowout preventer and remove it from the well, so they can then remove the capping stack that's kept oil from leaking since July 15 and replace the old blowout preventer with a new one.

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