The Silencers

Description for THE SILENCERS by Donald Hamilton:

Helm is sent to extract a female agent working as a stripper in a bar in Mexico. When she is killed before Helm can complete his mission to extract her, he finds himself teamed up with the woman's sister as he fights to save the lives of a group of scientists and American officials. Perhaps the most famous of the titles, due to the Dean Martin film of the same name, but where the movies were tongue-in-cheek, the novels are breathtakingly brutal.

This is #4 in Matt Helm Series.