Police have arrested five men they say were responsible for the arson attack at the Casino Royale in Monterrey that left 52 dead in one of the worst mass killings of ordinary Mexicans in the five-year-old campaign against organized crime.

The governor of the northern state of Nuevo Leon, where the firebombing occurred, said Monday that the five suspects said they were members of the Zetas, one of the most fearsome drug-smuggling and criminal gangs in Mexico, founded by former special forces soldiers. The gang is a major player in human trafficking, kidnapping, DVD piracy and extortion rackets.

Gov. Rodrigo Medina said all the suspects confessed to participating in the casino attack, as planners, perpetrators, lookouts or drivers.

In the immediate aftermath of the firebombing last week, President Felipe Calderon branded the attackers “true terrorists” and suggested that the country was entering a new stage in the drug war.

Calderon also blamed the United States — its drug users, its gun dealers and its government — for the tragedy, saying that the billions in illegal profits from drug sales fund the criminals in Mexico.

Also Monday, authorities released video footage from a security camera that showed some of the alleged assailants filling three cans at a gas station a few minutes before the attack.

Other video footage is more difficult to explain. One tape shows a group of Monterrey municipal police cars parked 200 yards from the casino as it is being torched. The police appear not to have responded. Images taken by residents using cellphones also show local police in the area before and during the attack.

As for the motive, Medina said the Zetas were extorting money from the casino. In Mexico, thousands of businesses must pay “tolls” and “rents” to criminals — or face the consequences.

Critics say the government is partly responsible for the tragedy, as the casino business has boomed under Calderon and many establishments operate without regulation or supervision. The operations are mostly cash businesses, ideal for money laundering. The Reforma newspaper reported Monday that 100 of the 800 casinos in operation are probably illegal.

Investigators are also interested in whether the Casino Royale had operating emergency exits and water sprinklers.

Video footage released earlier showed four vehicles with about a dozen men pulling up to the front of the casino Thursday afternoon; as some stand guard by their trucks and cars out front, others draw weapons and rush into the casino with the gasoline.

Less than three minutes later, black smoke and red flames are seen pouring out of the entertainment complex.

Monterrey residents marched on government offices Sunday and demanded Medina’s resignation. More than 1,500 federal police officers were sent to the city in a show of force.