George Stewart was a forensic technician fired from the Baltimore Police Department after Anthony DiNozzo reported him for contaminating blood samples.
Because of the error, a killer went free. Stewart later sued over his termination and won the ruling, but by then his name was mud and no-one else would hire him.
As a result, Stewart lost his wife, house and kids, forcing him to change his surname to Petri legally although he developed a strong grudge against DiNozzo for ruining his life, even referring to DiNozzo as a bastard.
Stewart/Petri was arrested after Gibbs found the body of a Jane Doe in his freezer without any legs.