Peter Clairmont was a serial killer responsible for the death of NCIS Special Agent Kensi Blye's father, Donald as well as members of the unit that both men were in.
While leading Blye and the other members of the sniper team, Peter got drunk one night and later in his drunken state and presumably consumed by rage, killed a man.
Peter covered his tracks and made sure no-one found out. He however did not get the whole job done as a reporter named Brad Stevens found out about this and was writing a story about it. Donald tried to warn him about Peter but before Brad could do anything, Peter killed him to cover his tracks and as a result, the story was buried.
Donald tried to turn Peter in with evidence of his guilt, but before he could so, Peter beat him to death and staged everything to make it look like Donald was killed by the car veering of course and Peter made it look like a drunken mistake. As such, Donald's body was left so disfigured that dental records had to be used to identify him.
After this, Peter enjoyed life as a free man until Donald's daughter, Kensi joined NCIS, seeking answers into her father's death and in order to cover his tracks he faked his own death and entered a new life as a private contractor under a false name: Peter Nelson.
Then a few years after Kensi joined NCIS, she began questioning all the men her father served with in order to find out who killed him. As a result he came out of hiding and started killing all the members of the team the same way he killed his targets when working for the unit.
While Owen thought Kensi herself was killing them, he killed everyone except his team's handler, Alex Harris. NCIS got to him first and arranged a meeting between him and Kensi.
Then unbeknownst to them Alex struck a deal with Peter to get Kensi in exchange for letting him live.
At the meeting Peter double-crossed Alex and killed him by shooting him and then tried to kill Kensi but failed due to the fact that it was revealed that she was wearing a bullet-proof vest which saved her.
Afterwards, Peter assembled his team of private contractors to help him kill Kensi and acquire the evidence that implicated him. He killed Megan Stevens and had one of his agents, Anya, go to her house and pretend to be her to get information out of Kensi.
Kensi, however, saw through the act and ended up killing the agent in self-defense and, once finished, called Peter from Anya's cell-phone, arranging a final showdown between her and Peter with the intention of settling their dispute once and for all.
Kensi eventually confronted him and after a brutal battle involving the two of them using hand-to-hand combat techniques, she prevailed, resulting in Clairmont being left injured with Kensi commenting to Callen, Sam and Granger that Clairmont would survive if he got to a hospital in time, and that she was finally done.
Just as she was walking away from the scene, Clairmont, enraged that he had been bested by Kensi, produced a .380 Walther PPK pistol, intending to shoot her but Owen Granger reacted and saved Kensi's life by shooting Clairmont, killing him instantly and shocking the rest of the team.
- Eighteen unknown targets (all killed and made to look like car accidents)
- Unnamed civilian- (killed in a drunken rage).
- Brad Stevens- (poisoned, made to look like a heart attack).
- Donald Blye- (beaten, rigged to look like a drunk driver accident).
- Patrick Phearson- (rigged to look like a car accident).
- Jason Clemp- (rigged to look like a car accident).
- Ray Duvont- (rigged to look like a car accident).
- David Blake- (rigged to look like a car accident).
- Jay Fisher- (died when his motorbike exploded)
- Alex Harris- (shot in the chest with a sniper rifle).
- Megan Stevens- (tortured and shot in forehead).
- Kensi Blye- (several times attempted).
- Jason Turner (incapacitated)
- Anya Fournier (deceased, killed by Kensi Blye)
- Dan King (deceased)
- Mario Cortec (deceased)
- Eric White (deceased)
- Victor Smith (deceased)