When an arson attack killed 52 people at a casino here last week, Mexico President Felipe Calderon called it the work of “true terrorists” and said U.S. drug users and gun dealers were partly responsible for his country’s violence.

Calderon cast the attack as a galvanizing moment in his administration’s battle against the cartels, characterizing the tragedy as a steep escalation of the conflict between security forces and gangsters who increasingly target civilians.

But a video and series of photographs showing the brother of Monterrey’s mayor receiving bundles of cash at a casino days before the massacre suggest its origins might lie in the old, familiar networks of corruption that have long plagued the country and nurtured the rise of organized crime.

It is not immediately clear what the payments were for or whether they were illegal. Nor is it clear whether there is any relationship between the payments taken by the mayor’s brother and the firebombing last Thursday of the Casino Royale.

But the videotape of the Monterrey mayor’s brother receiving wads of money raises in the public mind the possibility that the Zeta gangsters who carried out the attack were not terrorists but extortionists somehow linked to city hall.

A sensational video and series of photographs show Manuel Jonas Larrazabal, the brother of Monterrey mayor Fernando Larrazabal, receiving bundles of cash at a different casino just several days prior to the massacre. (Special To The Washington Post)

Mobbed by reporters, Monterrey Mayor Fernando Larrazabal — a member of Calderon’s National Action Party, or PAN — had no explanation for his brother’s appearance in the casino surveillance footage. His brother’s whereabouts are unknown.

“I am not responsible for what my brother did. I haven’t talked to him. If he’s committed a crime, then he needs to take responsibility before the law,” the mayor said.

An attorney for the mayor’s brother said Wednesday that his client had done nothing wrong and that the payments were for cheese and other food products provided to the casino, along with prize money.

“He’s a businessman, and he likes to have fun in casinos from time to time,” said the attorney, Jesus Martinez Garcia, according to Reforma, one of the newspapers that broke the story. “Some of his customers have asked him to come to the casinos for payment.”

Mexican authorities are operating under the theory that the Casino Royale was the victim of extortion and was torched by gunmen from the fearsome Zeta drug cartel because the owner, who has fled the country, was behind in paying bribes.

The politically explosive images appeared Wednesday morning on the front pages of the two most prominent newspapers in Mexico City and Monterrey and quickly went viral on competing media Web sites and on social media networks. Copies of the images were obtained separately by The Washington Post.

Investigators are focusing on the growth of casino gaming in Mexico operated by well-connected owners. These betting parlors and bingo halls — which operate mainly in cash, many illegally — are established centers of money laundering as well as fat targets for shakedowns, according to Mexico and U.S. law enforcement agents.

When Calderon took office in 2006, there were 198 casinos in Mexico. Today, the number of legal and illegal betting houses has grown to almost 800, according to the muckraking weekly magazine Proceso.

Mexican regulators held a news conference late Wednesday to address the growing scandal over loosely regulated gambling, saying most of the casinos were authorized under previous governments, not by the Calderon administration.

Here in Monterrey and the surrounding state of Nuevo Leon, there were five casinos a decade ago; now there are 58, according to state officials who said they are not sure how many have permits and how many are illegal establishments — and who their owners are. Licensing casinos is a federal responsibility.

The governor of Nuevo Leon, Rodrigo Medina, is calling for all casinos in the state to be shut down. He said Tuesday that the state was also investigating Monterrey’s municipal police force for possible ties to the casino attack.

Several city police patrol vehicles were spotted a block from the casino during and immediately after the attack. In security camera and cellphone videos, the officers do not respond.

Mayor Larrazabal and his older brother Manuel Jonas Larrazabal, 53, who is seen in the casino videotapes, are now subjects in the widening extortion and arson probe, said a top state official who spoke on the condition that he not be identified because of the sensitivity of the investigation.

Mexico officials are looking for the owner of the Casino Royale, Raul Rocha Cantu, and have solicited help from Interpol and the U.S. government, saying they think he is hiding in Dallas. An attorney for Rocha Cantu said his client is innocent but fears that he will be killed by gangsters if he returns and doubts that Mexico authorities can protect him.

Of the 52 people who died in the attack Thursday, 42 were women, including matrons who were burned at their bingo tables and overtaken by flames beside the slot machines. A pile of bodies was found, the victims asphyxiated, in the bathrooms where they tried to take refuge.

The Calderon government has responded to the attack by deploying 1,500 additional federal agents, adding to the almost 4,000 soldiers patrolling the streets of Monterrey, Mexico’s most important business and manufacturing hub.

The video images published in El Norte and Reforma newspapers Wednesday appear to be taken from overhead security cameras monitoring a casino floor on three occasions. Time stamps on the video suggest the tapes were made in May, June and August.

The videotape and still images show the mayor’s brother sitting at gaming tables, accepting and counting bundles of Mexican pesos. In one tape, Larrazabal puts the cash into a cardboard box; in another, he stuffs the money in his pocket.

The first video is dated May 30, five days after a convoy of 30 gunmen raided four casinos in the city. Several establishments were in legal disputes with the mayor’s office, including the Casino Royale.

Investigators say that the attacks were warnings from extortionists and that several other casinos were threatened with firebombing last week if they didn’t make their payments.

Five suspected members of the Zetas drug cartel are in custody in connection with the arson attack and were presented to the media on Tuesday in handcuffs. Authorities say the five suspects include planners, drivers, lookouts and perpetrators.State officials say they are seeking additional suspects and are working with intelligence units of the Mexican marines, because they do not trust local police.

One suspect denied that the gang intended to commit mass murder. According to Medina, the governor of Nuevo Leon, the suspects told police, “We just wanted to scare them. They called loudly to the people to leave, but things got out of control.”