Sullivan was an NCIS Special Agent assigned to the Office of Special Projects (OSP). Location was never Revealed.
Special Agent Sullivan was an NCIS Special Agent until he was killed in the line of duty in 1999 (the details regarding his death were never revealed). Sullivan had been personally selected and trained by Henrietta Lange for the OSP Team and his death left her devastated. As a result, she swore that Sullivan's death would be her first and last one as long as she was the Operations Manager.