Councilman Kevin Crossley is the murderer of Councilman Richard Durban. He was motivated by his district lines being changed during redistricting, which Crossley feared would get him kicked out of office. Crossley was arrested once his motives, his coat, and a weapon he stole when he was a cop were all discovered.
Crossley was released after a mistrial was declared when he slandered Durban posthumously, as well as a mea culpa that Logan testified to on the stand when each side tried to blackmail a witness. When Crossley goaded Logan after his release, Logan punched him, which resulted in his exile to Staten Island. (L&O: "Pride")