Pacci's career as an NCIS Special Agent came to an abrupt and very tragic end when he was killed in the line of duty in April 2004.
Not much is known about Pacci or where he came from but that he eventually joined NCIS, presumably working his way through the ranks before he became an official NCIS Special Agent.
At one point he trained NCIS Agent Cassie Yates.
Prior to his death, Pacci also investigated the disappearance of Navy Petty Officer Third Class Carolyn Figgus but was ultimately forced to stop working on the case after it went cold due to any lack of possible leads or new evidence.
It is also mentioned in the episode SWAK (episode) that Pacci was the investigator for a rape case from February 2001.
NCIS Season 1
NCIS Season 2
NCIS Season 8
NCIS Season 12
Pacci's murder left his NCIS colleagues devastated and greatly shaken with Gibbs being personally affected as during their last meeting in the NCIS Season 1 episode, UnSEALeD (episode), Pacci had approached him, asking for help in regards to a case but Gibbs who along with his team was searching for Jack Curtin was forced to turn Pacci down.
In spite of this, the NCIS Major Case Response Team was able to solve the case with Gibbs avenging Pacci by shooting his killer, Amanda Reed/former Navy Lieutenant Commander Hamilton Voss dead in a bar in Georgetown when Reed/Voss attempted to escape.