The fire at the Casino Royale in Monterrey, a city that has seen a surge in drug cartel-related violence, represented one of the deadliest attacks on an entertainment center in Mexico since President Felipe Calderon launched an offensive against drug cartels in late 2006.
"This is a night of sadness for Mexico," federal security spokesman Alejandro Poire said in a televised address. "These unspeakable acts of terror will not go unpunished."
Calderon tweeted that the attack was "an abhorrent act of terror and barbarism" that requires "all of us to persevere in the fight against these unscrupulous criminal bands."
Nuevo Leon state Gov. Rodrigo Medina told the Televisa network that 53 people had been confirmed dead in the attack.
"But we could find more," said state Attorney General Leon Adrian de la Garza, adding that a drug cartel was apparently responsible for the attack. Cartels often extort casinos and other businesses, threatening to attack them or burn them to the ground if they refuse to pay.
State police officials quoted survivors as saying armed men burst into the casino, apparently to rob it, and began dousing the premises with fuel from tanks they brought with them. The officials were not authorized to be quoted by name for security reasons. De la Garza said the liquid appeared to be gasoline.
With shouts and profanities, the attackers told the customers and employees to get out. But many terrified customers and employees fled further inside the building, where they died trapped amid the flames and thick smoke that soon billowed out of the building.
Workers continuing to remove bodies well into the night.
Monterrey Mayor Fernando Larrazabal said many of the bodies were found inside the casino's bathrooms, where employees and customers had locked themselves to escape the gunmen.
In an act of desperation, authorities commandeered backhoes from a nearby construction site to break into the casino's walls to try to reach the people trapped inside.
Maria Tomas Navarro, 42, stood weeping at the edge of the police tape stretched in front of the smoke-stained casino building. She was hoping for word of her brother, 25-year-old Genaro Navarro Vega, who had worked in the casino's bingo area.
Navarro said she tried calling her brother's cell phone. "But he doesn't answer. I don't know what is happening," she said. "There is nobody to ask."
Larrazabal said the casino, in a well-off part of Monterrey, had been closed by authorities in May for building an expansion without a permit, but a judge later granted the owner an injunction to continue operating.
Initial reports said 11 people had been killed, but the death toll climbed as emergency personnel and firefighters searched the casino building. Medics treated survivors for smoke inhalation.
State police officials initially said witnesses reported hearing three explosions before the fire started, but later said a flammable material was used. The officials were not authorized to be quoted by name for security reasons.
The reports of explosions may have been the sound of the ignition of the liquid.
It was the second time in three months that the Casino Royale was targeted. Gunmen struck it and three other casinos on May 25, when the gunmen sprayed the Casino Royale with bullets, but no was reported injured in that attack.
Last month, gunmen killed 20 people at a bar in Monterrey. The attackers sprayed the bar with rounds from assault rifles, and police later found bags of drugs at the bar.
Monterrey has seen bloody turf battles between the Zetas and Gulf cartels in recent months. Once Mexico's symbol of development and prosperity, the city is seeing this year's drug-related murders on a pace to double last year's and triple those of the year before.
Associated Press Writer Katherine Corcoran contributed to this report.
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