MEXICO: A shooting that killed eight people partying on a seaside boulevard in northwestern Mexico may have been the work of vigilantes targeting car thieves, a prosecutor said.

The gunmen drove up in a white SUV and fired on the festive crowd without saying a word, Sinaloa state deputy Attorney General Jose Luis Leyva said Sunday.

A 15-year-old boy and a 17-year-old girl were among the youngest killed Saturday night in the Pacific coast town of Navolato. Four people were wounded.

Also among the dead were two brothers in their 30s who had a record of car theft, Leyva told The Associated Press. He said investigators were considering the possibility that the gunmen belonged to a criminal gang known as the "Death Squad," which has been killing car thieves in the region.

"It's one line of the investigation, and of course we are going to follow it," Levya said.

He said ballistic evidence suggested that some people may have fired back at the gunmen.

Sinaloa, a state straddling Mexico's northern Pacific coast, is cradle of at least two main drug cartels and a longtime hotspot for gang violence.

Increasingly, civilians have been caught in the crossfire of Mexico's relentless gang violence. Gunmen apparently targeting rivals have sprayed bullets into clubs, bars and taco stands. Last year, a grenade attack killed eight people and wounded 106 during an Independence Day celebration in the western city of Morelia.

The seaside boulevard in Navolato is a popular gathering place for young people on the weekends, but it was nearly deserted the day after the attack.

Gang violence has claimed at least 13,500 lives since President Felipe Calderon took office in 2006 and sent tens of thousands of troops and federal police to battle drug cartels across Mexico.

Meanwhile, authorities arrested four men accused of killing at least 211 people for the Juarez cartel. Most of the killings took place in Ciudad Juarez, a city across the border from El Paso, Texas, according to a statement from the joint military and police force in charge of security in the city.

The four men allegedly belong to La Linea, a gang of hit men for the Juarez cartel, the statement said. One of the men alone is accused of killing 97 people and another 87.

Security forces caught the men based on information from seven suspects arrested last week for allegedly killing a total of 39 people, the statement said.

Authorities gave no information on the victims or a timeframe for the killings. Vladimir Tuexi, a spokesman for the Chihuahua state attorney general's office, said there would no further comment for the time being to avoid undermining the investigation. Officials say the men confessed to the killings.

Ciudad Juarez is Mexico's most violent city, with more than 1,400 people killed this year alone despite the presence of thousands of soldiers and federal police. The vast majority of homicides are unresolved.

In Tijuana, meanwhile, about 100 relatives of missing people demonstrated at site where a man allegedly helped a top druglord dispose of 300 bodies over several years by dissolving them in a corrosive substance.

The protesters accused federal authorities of ignoring their requests to show pictures of their missing loved ones to Santiago Meza, who was arrested in January. They also accused Attorney General Eduardo Medina-Mora of doing nothing to search the site for remains, and threatened to start digging themselves if no steps were taken in a month.

"Since this site was discovered it has remained the same. There has been no excavation, nothing has been moved," said Fernando Osegueda, whose son disappeared in 2007. "But if nothing changes in a month, we are going to come back and excavate and find the remains."

The Attorney General's Office did not immediately respond to requests for comment.