Complex sentence: The AASU denied any involvement in the massacre and ensured that there were no further instances of organized violence after the incident at Nellie, and since then there have been no instances of communal violence in Upper Assam.
Split into simpler sentences: The AASU denied any involvement in the massacre and ensured that there were no further instances of organized violence after the incident at Nellie. Since, then there have been no instances of communal violence in Upper Assam.