The announcement went out on Facebook, an open invitation summoning revelers to a birthday party at the Italia Inn in the northern Mexico city of Torreon.
But early Sunday, as musicians serenaded amid food, drink and dancing, gunmen burst into the party, blocked the exits and, without saying a word, opened fire, killing 17 men and women and injuring 18.
It was one of the highest single-incident death tolls since the beginning of Mexico's raging drug war, which has claimed nearly 25,000 lives from the time President Felipe Calderon launched a military-led offensive against powerful narcotics cartels in December 2006.
The dead identified by Sunday afternoon were in their 20s and 30s, some related to one another, according to the prosecutor's office for the state of Coahuila, where Torreon is located. Coahuila borders Texas, and while plagued with the same violence terrorizing other border states, has received less publicity.
Authorities confirmed the number of dead and said some of the injured were in critical condition.
Pictures from the scene showed toppled white plastic chairs, scattered musical instruments and cups, plates and shoes strewn on a blood-splattered floor alongside a tented pool. Empty beer bottles were lined up on tabletops, and bullet holes punctured the walls.
"The party was ongoing … when gunmen arrived in several vehicles, disembarked and, without uttering a word to those in attendance, opened fire," Jesus Torres, state attorney general, said in a statement. The assailants fled.
Investigators recovered 122 spent shells from high-velocity assault rifles.
Many of the partygoers at the event that began Saturday evening were attracted by an invitation that appeared on Facebook, according to reporters at the Torreon edition of Milenio newspaper and television. The posting included directions and a map.
Organizers of the party were a gay group, Milenio said, but the invitation specifically said the event was open to all. Although that raised the possibility of an antigay hate crime, most speculation centered on drug traffickers as the assailants.
Violence has surged in Coahuila, a coveted smuggling transit point, and neighboring states as the longtime dominant Gulf and Sinaloa cartels battle the paramilitary Zetas. Mass shootings at clubs, bars, rehab clinics and other sites are now common in once-calm cities such as Torreon, as well as in other, more troubled neighboring municipalities.
The government attributes much of the rise in violence to infighting among drug gangs, whose leadership has been splintered after the arrest of kingpins.
Federal police said in a statement Sunday that they have arrested 1,626 people suspected of belonging to the command structures of Mexico's drug gangs since Calderon launched his offensive. They said 622 of the detainees belong to the Gulf cartel and 304 to the Sinaloa cartel.
On Sunday, a judge formally charged an alleged leader of the Beltran Leyva cartel, Jose Gerardo Alvarez, with organized crime.
Alvarez, who had a $2 million U.S. bounty on his head, was captured in April after soldiers battled his men in a wealthy neighborhood on the outskirts of Mexico City.
Information from the Associated Press was used in this report.