Five men have been arrested in connection with an attack last week on a casino in the northern city of Monterrey that killed 52 people, the state governor of Nuevo Leon, Rodrigo Medina, said on Monday.
Mr. Medina said the suspects were believed to be members of the Zetas drug cartel, which has ravaged the state and other parts of Mexico with killing, kidnapping and extortion.
"From the initial indications we've observed the target was the casino, not the civilian population," the governor told a televised news conference. He added that one strong line of investigation was that the casino was being subjected to extortion and that the search for suspects was continuing.
President Felipe Calderon declared three days of national mourning after the attack and in a television interview on Monday pledged to continue the fight against organized crime.
Violence has leapt in prosperous Monterrey since Mr. Medina became governor, and hundreds of demonstrators staged a protest outside his office on Sunday, calling on him to resign.
The situation in Nuevo Leon has coincided with a surge in violence across Mexico since Mr. Calderon sent in the army to crush the powerful cartels when he took office in December 2006.
Monterrey, the state capital of Nuevo Leon, was once seen as a paragon of economic development in Latin America. But the escalating violence has sparked concerns that both the city and the country are losing the fight against organized crime.
The Mexican Attorney General's office on Friday offered a reward of $2.4 million for information leading to the arrest of perpetrators of Thursday's attack.