Justice Denied by Robert K. Tanenbaum

From the publisher (source: The Internet Book List):
In Justice Denied, Butch Karp is newly appointed Homicide Bureau Chief of the New York District Attorney's office. Karp, the prosecutor with a perfect record of guilty verdicts, finds himself entangled in a case of murder that could be the first he'll ever lose - and the last he'll ever try. A Turkish diplomat at the United Nations is brutally gunned down on a Manhattan street. The mayor's office demands immediate results from the D.A.'s office. A young and ambitious assistant D.A. homes in quickly on an unassuming Armenian, a member of an extremist anti-Turk sect. But Butch Karp knows that the investigation has been more expedient than thorough. Karp launches his own parallel investigation, joining forces with his wife, the cynical but ferociously dedicated Assistant D.A. Marlene Ciampi, and Harry Bello, the street-callous detective who can grill a suspect with the same sharp and silky aplomb with which he can melt into a doorway on a tail job. Working on a lead from a behind-the-scenes Armenian power broker, Karp finds himself at the heart of an ancient conflict between Armenian and Turk, bafflingly obscure to Westerners, bloodily serious to its participants. He embarks on the trail of a priceless stolen relic hotly contested by Turks and Armenians for centuries. Suspects on both sides of the law multiply at a dizzying pace. More victims die, and for Karp some are frighteningly close to home. Though the black hand of the Mafia casts its shadow on the mystifying chain of events, every instinct Karp has tells him to press on. But now the case has spread its lethal menace over his wife, Marlene, and their infant daughter. And Karp realizes that to keep digging is to go where even the steeliest-skinned, most single-minded cops fear to tread.

This book is part of the "Butch Karp" series.
Here are some other books from this series: