Hitmen working for a drug gang are the main suspects in the weekend slaying of 16 young partygoers in Mexico's Ciudad Juarez, an official said Tuesday, as families prepared to bury their dead.

Some 15 gunmen opened fire on a party packed with teenagers in the early hours of Sunday, in a particularly extreme attack in the northern border city which is notorious for deadly score-settling between drug gangs.

The majority of the dead were under 20, and most were high school or college students.

Some families placed altars with photographs of their deceased children and religious images in front of their homes ahead of Tuesday's funeral ceremonies.

"They were only at a party. They hadn't harmed anyone," one distraught neighbor, Liliana Reyes, told journalists outside.

More than 40 people died in attacks in northern regions during the long holiday weekend, but the shooting at a party of some 60 youngsters sparked widespread outrage.

Lawmakers in Congress afterwards called for a special session with top government ministers to discuss the spiraling violence.

Suspected drug attacks have marked Mexico in the past three years, with more than 15,000 killed since President Felipe Calderon launched a military clampdown on organized crime.

Calderon condemned the "cowardly assassination" of the partygoers during a visit to Japan, and authorities offered a reward of one million pesos (77,000 dollars) for information on the perpetrators.

An official from the Chihuahua state prosecutor's office, who declined to be named, said the Los Aztecas group of hitmen were the main suspects.

Los Aztecas work for the Juarez gang, which is fighting a bloody turf war against the Sinaloa cartel in the country's most violent city of Ciudad Juarez, which saw some 2,660 murders in 2009 alone, more than seven a day on average.

Authorities suspect that the party slayings were linked to a revenge attack.

"The hitmen were aiming at someone specific, according to witness statements," the state official said, adding that an unidentified man had warned the youths of the attack beforehand.

President Calderon has scored several high-profile victories against drug gangs in recent months, including the slaying of kingpin Arturo Beltran Leyva in December, and the arrest of the notorious hitman Teodoro Garcia Simental, known as "El Teo," in January.

But violent attacks remain part of daily life in some areas, particularly around the northern border with the United States.

Gunmen killed 10 people over the weekend in Torreon, in the northern state of Coahuila, after opening fire on a crowd inside a bar.

Armed attackers on Monday opened fire in a bar in Ciudad Juarez, killing five people and injuring six others, police said.

Federal police also confronted suspected members of the Los Zetas gang in two successive shootouts late Monday, which left seven traffickers and one policeman dead in Torreon.