Serpent Gate by Michael McGarrity

After receiving a call from the newly appointed chief of the New Mexico State Police, ex-Santa Fe chief of detectives Kevin Kerney is thrown into an investigation of a small-town cop-killing no one has been able to solve. His only lead: a homeless schizophrenic's ramblings about rape and an uncharted place called Serpent Gate.
Meanwhile, back in Santa Fe, priceless art is stolen from the governor's offices and a beautiful young blonde is murdered in a millionaire's mansion. Kerney follows a trail of clues to Mexico, where he faces off against an old nemesis with powerful government connections. Unwilling to back down, Kerney must use all of his tenacity, raw courage, and knowledge of the criminal mind in a bloody showdown that may cost him his life.