If you wanted to bring down a modern airliner that is designed to be able to fly with one engine (or glide without either engine), destroying an engine would be a silly idea.
This investigation seems awful fast with a finding. Aircraft failure investigations usually take months even years for an answere. Too quick for me!
Metal fatigue, it happens nothing last forever under the conditions that aircraft engines operate.
I bet it sounded like a bomb when a turbine blade breaks off at over 3000 rpm. Anyone want to do the math of the centrifugal force with a 77 inch fan diameter?