Death toll hits 18 in party massacre in northern Mexican city plagued by drug violence

Gunmen massacre 18 partygoers in northern Mexico

Massacre of 18 at party in northern Mexico the latest in string of mass killings

Officials say gunmen kill 18 at party in Mexico

Massacre of 17 at party in northern Mexico the latest in string of mass killings

Officials say gunmen kill 17 at party in Mexico