A Bullet for the Cartel by Lawrence W. Erickson

Dispatched by the leader of a Mexican cartel, hired assassins descend on the City of Roses. They are to locate and eliminate the only surviving family member of a traitor found within the cartel organization. The target's last hope of escaping torture and death is Portland private investigator Nate Harver and his partner Donnie Shepard. The target is in FBI custody and hiding in Portland, Oregon. The danger is compounded by a leak to the cartel from within the FBI. The assassins' initial attempt is thwarted only by Nate's chance intervention. As Nate and the target form a bond stronger than the horrors they face, the mayhem reaches a violent crescendo. Facing the cartel and their own personal terrors, Nate and Donnie go on a ruthless offensive. The body count rises as the suspense builds from the Pacific Northwest through the Caribbean and Venezuela, with an explosive climax in the Gulf of California.