There may be an official report somewhere, but since I don't read Spanish I haven't found it. The investigation is the responsibilty of the Uruguayan Air Force since there's no civil equivalent.
From various other sites it seems the FDR recorded an airspeed increase from 200 to 450kts in 4 seconds, and ATC tapes show a 4600fpm descent until mode C was lost at FL257. One site says impact occurred 1 minute later (!) although it does not say how this was established.
Given the nasty Cb reported at the time, my guess is an inflight breakup or loss of hull integrity. And it is just a guess - I've seen no wreckage diagrams to support it.