Detective Mark Rivera is an NYPD narcotics detective with a blackmailed teenage informant in Kevin Stanton. In an attempt to catch Reggie Mathis and Oscar Benvenidez, Mathis shot Kevin and a girl with him, Sara Teasdale, Sara dying from the gunfire.
When it was revealed Rivera not only deliberately endangered Kevin with his assignment, but he turned away a 911 dispatch Benvenidez called for Kevin and Sara, he was charged with manslaughter to force his testimony against the two dealers.
Rivera pleaded down to a lesser charge in exchange for no jail time, but his career was effectively over from the conviction. (L&O: "Bait")