Gunmen stormed a party in northern Mexico and massacred 17 people, authorities said.
The assailants arrived at the gathering in the city of Torreon in several cars and opened fire without saying a word, the Coahuila state Attorney General's Office said in a statement.
At least 18 people were wounded.
Several of the victims were young and some were women, but their identities and ages were not yet determined.
Television footage showed the patio of the house streaked with bloodstains and white plastic chairs overturned beneath a party tent decorated with pictures of snowmen.
Police found more than 120 bullet casings at the scene, most of them from .223-calibre weapons.
Investigators had no suspects or information on a possible motive.
Coahuila is among several northern states that saw a spike in drug-related violence that authorities attribute to a fight between the Gulf cartel and its former enforcers, known as the Zetas.
In May, gunmen killed eight people at a bar in Torreon. Later that month, a television station and the offices of a local newspaper came under fire. A pregnant woman was wounded in the attack on the offices of Noticias de El Sol de la Laguna.
Across northern Mexico, there have been increasing reports of mass shootings at parties, bars and rehab clinics.
In the worst such massacre this year, gunmen raided a drug-rehab centre in the northern city of Chihuahua and killed 19 people last month.
In January, gunmen barged into a private party in the border city of Ciudad Juarez and killed 15, many of them high school or university students.
Relatives said the January attack was a case of mistaken identity, while state officials claim someone at the party was targeted, although they have not said who it was.
The killings in Torreon came three days after the first successful car bombing by drug cartels, an attack that introduced a new threat to Mexico's raging drug war.