how to find the session details of a killed transaction that is being rolled back

how to find the  session details of a killed transaction that is being rolled back

We see that some session was killed and it triggered rollback

we used below query

select usn, inst_id,state, undoblockstotal "Total", undoblocksdone "Done", undoblockstotal-undoblocksdone "ToDo",

    decode(cputime,0,'unknown',sysdate+(((undoblockstotal-undoblocksdone) / (undoblocksdone / cputime)) / 86400)) "Estimated time to complete"

    from gv$fast_start_transactions;

We want to identify which sid was running at the time of termination