MONTERREY, Mexico (AP) ― Two dozen gunmen burst into a casino in northern Mexico on Thursday, doused it with gasoline and started a fire that trapped gamblers inside, killing 53 people and injuring a dozen more, authorities said.

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.