One report can be dismissed as potentially unreliable but
Reuters report finding several people providing similar accounts, namely 'fire' (sparks, smoke) from the rear of the plane, spilling luggage and a "strange rattling noise" both prior to impact with terrain. If they truly are independent witness reports then that could be significant. However, equally, the eye-witnesses could mistakenly be reporting "white smoke" when actually observing vapour trailing or fuel dumping.
The lack of ADS-B data after 6 minutes into the flight, until impact well over 30km of travel (several minutes) later, not only illustrates ADS-B coverage limits but points to a lack of altitude for the remaining flight duration otherwise it would re-appear above the local ADS-B receiver(s) horizon(s).
These factors might encourage investigators to rule out in-flight fire, small incendiary/explosive device or contributory engine issue (HAAB is relatively high altitude and tends to be hot which will contribute to lack of lift at take off).