The death toll rose to 18 on Monday, a day after gunmen burst into a birthday party and opened fire in an attack that was violent even by the bloody standards of Mexico’s drug war. The government said the attack, at a party gathering in the northern city of Torreón, appeared to be the work of a drug gang, but officials said they had not determined the possible motive for the killings as of late Sunday. Among the dead were the birthday honoree, who was identified on Monday as Hector José Mota Mendez, and five women. The gunmen, who struck around 1:30 a.m., were traveling in a convoy of eight cars, witnesses told local news outlets. Without warning they entered the party and began firing indiscriminately before escaping. “They shot anything that moved,” according to a local police source quoted in the newspaper El Norte. Although the vast majority of the nearly 25,000 people killed since President Felipe Calderón began his attack on drug gangs in December 2006 have been men, women have increasingly become targets. Torreón, in the state of Coahuila, which borders Texas, has become a battleground in the drug war as a transit point to the United States. At the end of January, gunmen killed 10 young people in an attack in a bar there. In May, eight more young people were killed in an attack in another Torreón bar. Several of those who were killed were students and did not appear to have any links to drugs. In May 2009, a journalist from Torreón was abducted and killed by kidnappers that investigators suspect were members of the Zetas drug gang. The birthday party killings in Torreón came three days after a car bomb in the border city of Ciudad Juárez killed four people, including two federal policemen. It was the first time that a car bomb had been used in an attack in the drug violence, leading to fears that Mexico may be facing a new kind of terrorism.