Gunmen opened fire on a group of people gathered on a soccer field here, bringing to 14 the total number killed in three separate attacks in northern Mexico, officials said. The attacks on Tuesday were the latest in a wave of deadly shootings that have gripped the country, where some 37,000 people have been killed in largely drug-related violence since 2006. The soccer field attack was carried out in Ciudad Juarez -- Mexico\'s most violent city -- by unknown gunmen wielding AK-47 assault rifles, a police source said. Earlier in the day, five people between the ages of 15 and 20 were gunned down by gunmen riding in two vans in Santa Catarina, near the northern city of Monterrey, Nuevo Leon state security spokesman Jorge Domene said. In the third attack, gunmen shot dead four people in a working-class district of Monterrey itself, Domene said. It was not immediately clear who was behind the attacks, but Monterrey has recently seen a rash of violence as the Gulf cartel and its former allies the Zetas have battled over territory. The massive wave of violence struck four years ago, when President Felipe Calderon deployed some 50,000 troops in a military crackdown on several powerful cartels already embroiled in brutal turf wars.