A Mexican official says police have arrested five suspects in an arson attack on a casino in northern Mexico that killed 52 people last week.

Nuevo Leon state Governor Rodrigo Medina said Monday the five suspects are thought to be members of the Zetas drug cartel, one of the most violent in Mexico. He said security forces also are looking for other people involved in the arson attack on the Casino Royale in the city of Monterrey.

A group of assailants carrying a flammable liquid burst into the building last Thursday and set it on fire.

Authorities suspect the attack was in retaliation for the casino owners' refusal to pay extortion money. They are offering a $2.4 million reward for information leading to the arrest of the perpetrators.

Organized and drug-related crime in Mexico has killed about 35,000 people since late 2006, when President Felipe Calderon began a military offensive against the country's drug cartels.