Slow motion film footage released by Space X (Space Exploration Technologies) appears to show the Merlin 1c engine "letting go" emitting hot gases and eventualy causing debris to fall away. SpaceX notes that the engine stayed intact and the debris was engine fairing material.
Comment by David Todd: While having so many engines on a first stage actually increases the chance there being at least one engine failure on a flight (the reason Falcon 9 have nine engines is purely because their Merlin 1c engine is actually undersized for this role), it does actually increase mission redundancy (well so long as the other engines are not affected by the failed/shutdown engine). In other words, the remaining working engines can compensate for the loss of thrust.
While this just about happened for the rocket in getting its main Dragon payload to the right orbit, the secondary Orbcomm was not so lucky and it is how stranded in a useless orbit. Nevertheless, the fact that the rocket continued to fly after this event is a testament to rhe robustness and redundancy of the vehicle, and to its physical protection systems between the engines which kept the other engines running, SpaceX and NASA, will however, still have the headache of the delay that any investigation into this event will cause to their launch schedules.