Juan Pablo Vazquez, 20, was arrested earlier this month on charges that included the slayings of a traffic police officer and dealing cocaine and marijuana in the violence-stricken northern Mexico state.
Monterrey has been the epicenter of a struggle for dominance between two violent drug cartels since 2010. CNN said the city, capital of the state of Nuevo Leon, has recorded 223 homicides so far this year.
Police sources told CNN Vazquez was a gunman for the Gulf cartel, which has been warring with the equally murderous Zetas. Police said he had confessed to 45 of the killings.
CNN noted police did not consider Vazquez to be a particularly prolific contributor to the death toll notched by the Gulf cartel in recent years.