Hundreds of soldiers and federal agents are raiding casinos in this northern city, authorities said Saturday, two days after an arson attack on a gambling house killed 52 people and stunned a country that had become numb to massacres and beheadings.

Security forces had so far confiscated about 1,500 slot machines at 11 casinos in Monterrey and its surroundings and arrested three people, Mexico's tax agency said. It said the continuing operation was meant to verify whether casinos had paid taxes or introduced slot machines illegally.

This was different

Mexicans have endured plenty of horrific crimes during their country's five-year offensive against cartels.

However, the casino attack had a major impact because many of the victims were from the middle class, not cartel foot soldiers or migrants who are the usual targets, said Jorge Chabat, an expert in safety and drug trafficking at the Center for Research and Teaching in Economics.

"We're talking about an attack on a civilian population of a certain income," he said. "Because who was there was from the middle class, the upper middle class of an important city in Mexico."

Thursday's arson attack by gunmen was a macabre milestone in a conflict that the government says has claimed more than 35,000 lives since President Felipe Calderon launched an offensive against drug cartels in late 2006. Others put the death toll near 40,000.

The torching of the Casino Royale has raised questions over Mexico's regulatory controls for fast-spreading gambling houses.

Authorities have not been able to reach the owners of two companies pointed out as titleholders of the casino. Jorge Domene, security spokesman for Nuevo Leon state, said an order to appear before state police has been issued for owners of the two companies, CYMSA Corp. and Vallarta Attractions and Emotions.

Casino Royale's legal representative, Juan Gomez, told reporters the shareholders of the business were Jorge Alberto and Raul Rocha Cantu. They will meet with police when authorities set the time, Gomez said. Their family members have been prohibited from leaving the city, he said without offering details.

A soldier holds a machine gun outside a casino in Monterrey, Mexico, on Saturday.A soldier holds a machine gun outside a casino in Monterrey, Mexico, on Saturday. (Arnulfo Franco/Associated Press)During the raids, which began Friday, about 700 soldiers, federal police and Treasury Department agents seized slot machines and put them in moving trucks.

Authorities did not say the raids were related to the arson. But one of the casinos searched was also registered under Vallarta Attractions and Emotions, according to the gaming unit of Mexico's Interior Department. Information of the other locations was not immediately available.

Federal police spokesman Juan Carlos Buenrostro said additional security forces were being deployed to this industrial metropolis of more than four million people Saturday. Buenrostro did not specify the actions police would carry out or the number of agents who would take part.

Mayor Fernando Larrazabal said the Casino Royale and other 12 casinos violated municipal laws and were allowed to remain open after obtaining federal court injunctions.

The casino had been attacked twice before, including an incident in May when gunmen strafed its exterior. Last month, gunmen killed 20 people at a bar in Monterrey.

'We are facing true terrorists who have gone beyond all limits.'— President Felipe Calderon

Cartels often extort casinos and other businesses, threatening to attack them or burn them to the ground if they refuse to pay. But Gomez, the owners' representative, said the Casino Royale had not received extortion threats.

Speaking at a news conference televized by Milenio Television, he also said the casino had the appropriate permits and met safety standards.

Authorities have not blamed a specific drug-trafficking organization for the casino attack. But the city has been ensnared in a turf battle between the Gulf cartel and its offshoot, the Zetas, and is on track for record levels of killings this year.

A surveillance tape showed the Casino Royale building engulfed in flames in little more than two minutes after eight or nine men arrived in four cars carrying canisters into the building.

Bodies found in offices

Authorities said they were still investigating whether the casino's emergency exits were blocked. But many bodies were found in offices and the bathrooms, indicating the victims were expecting a shootout.

"They sought places to protect themselves from firearms," said Jorge Camacho Rincon, the state civil protection director. "They went running to closed areas."

Most died of smoke inhalation and were found clutching cellphones, a law enforcement official who wasn't authorized to be quoted by name told The Associated Press.

Saturday was the second day of mourning declared by Calderon, who labelled the attack the worst against civilians in the nation's recent history.

"We are facing true terrorists who have gone beyond all limits," Calderon said.

Calderon is offering a $2.4-million US reward for information leading to the capture of the casino's attackers, the same amount offered for the arrest of top drug lords. Authorities had sketches of three of the men based on interviews with 16 survivors of the fire.