PIEDRAS NEGRAS, Mexico -- The gunmen did not say a word as they jumped from their cars and stormed the private party. They simply opened fire. When they were done, 18 people were killed.
Sunday's massacre in the city of Torreon was ghastly, but no longer unprecedented in northern Mexico, a region that is slammed day after day by gruesome murders that authorities attribute to an increasingly brutal battle between drug gangs feuding over territory.
Investigators had no suspects or information on a possible motive in the attack, but Coahuila, where Torreon is located, is among several northern Mexican states that have seen a spike in drug-related violence as the Gulf cartel and its former enforcers, the Zetas, fight for control of drug-trafficking routes.
The attack on the party came just three days after a car bomb killed several people in the northern city of Ciudad Juarez -- and a little more than a month after assailants raided a drug-rehab center in the northern city of Chihuahua, killing 19 people in cold blood.
Photos of the aftermath showed the patio of the house in Torreon streaked with bloodstains and white plastic chairs overturned beneath a party tent decorated with pictures of snowmen.
The Coahuila state Attorney General's Office said in a statement Monday that the death toll rose to 18 overnight after one of the wounded died. There were 12 male and six female victims; among them were four teenagers, the youngest a 17-year-old boy. Seventeen were wounded.
The assailants arrived in a convoy of vehicles. Police found more than 120 bullet casings at the scene, most of them from .223-caliber weapons.
Torreon is no stranger to violence.
In May, gunmen killed eight people at a bar in the city, while 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 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 say that 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.
On Thursday, drug-gang members set off their first successful car bomb. They lured federal police and paramedics to an intersection in Ciudad Juarez by calling in a false report of a wounded police officer, and when the authorities were in place at the scene, they detonated the explosive. Three people were killed, including a federal officer and a private doctor who had rushed to help.
The FBI has sent a small team to the crime scene to offer technical assistance to the Mexican investigators, FBI spokeswoman Andrea Simmons said. She did not immediately offer more details. Mexican investigators have not said what type of explosive was used.
Officials say 24,800 people have been killed in drug-gang violence since President Felipe Calderon declared war on the cartels in December 2006, deploying soldiers and federal police to fight traffickers.
On Saturday, four municipal police officers were ambushed and killed in the Pacific coast resort city of Acapulco, state police said.
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 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 in a wealthy neighborhood on the outskirts of Mexico City.
The federal government has steadily wiped out the leadership of the once-powerful Beltran Leyva cartel. In December, cartel boss Arturo Beltran Leyva was killed in a gunbattle. Two of his brothers are behind bars.
A fourth brother, Hector Beltran Leyva, remains at large and is believed to be battling for control of the cartel against Edgar Valdez Villareal, a U.S.-born suspect known as "La Barbie."
Mexican authorities say Alvarez partnered with Valdez in his quest for control of the gang.