MEXICO CITY - Gunmen stormed a birthday party in embattled Ciudad Juarez and killed 13 young people, the latest outrage in a city crippled by warring drug gangs.

State Attorney General Carlos Salas told reporters at a news conference at the crime scene that the dead ranged in age from 13 to 32 and included six women and girls. A dozen people were wounded in the attack late Friday in a lower-middle-class barrio of Juarez.

Salas said that investigators found 70 bullet casings at the scene and that the attackers escaped. He offered no possible motive.

Gunmen killed 15 people at a birthday party attended by teenage football players in a similar attack in the city in January.

The deaths captured the attention of the nation and forced President Felipe Calderon to promise to restore order. At public meetings back then in Juarez, he was heckled by audiences and confronted by the angry mothers of the dead.

Calderon, supported by U.S. Ambassador Carlos Pascual, committed large investments to mend the tattered social fabric of the city by building schools, parks and clinics and offering work to the tens of thousands of underemployed youth in program called "We Are All Juarez."

The Mexican government also vowed to replace military troops, which had been in the streets since January 2008, with federal police.

Federal police are in the leadership position in the city, but army troops actively patrol the city. Social spending has soared, but most projects have not been completed.

The Juarez surge, as it has been called by state security experts, has largely failed to calm the violence. Many days, a dozen people are shot dead in Juarez - in cafes, bars, hospitals and parks but mostly on the street. More than 2,500 people have been killed this year.

Mexico's secretary of the interior, Francisco Blake Mora, condemned the killings in a statement and promised to help the families of the victims and investigate the crimes.

Calderon, in a statement made through his Twitter account, said his government was outraged by the deaths and "expresses its strongest protest against the murder of the young people in Ciudad Juarez."