Corrected entry: The electromagnet locking down the vault could not have simply deactivated if the power was cut. The steel used in the construction of the vault would have developed into a permanent magnet (remanence) thus preventing the vault from being opened. Removing this remanent magnetism is very difficult, one of the simpler ways is to heat the material to its curie point, which around 1500 degrees Fahrenheit (816 degrees Celsius) in 9.0% carbon steel, the terrorists did not do this or any other method to remove the remanent magnetism.
Correction: If the electromagnetic lock instantly turned the steel vault into a permanent magnet, then it would render the lock and the vault useless. No one could ever enter once the lock was activated. Electromagnetic locks are used in many high security areas and that is how they work: Turn off the power to the magnet and the door opens.