Federal police arrested a state police officer in connection with last week's arson fire at a casino that killed 52 people in northern Mexico.
The state police agent was caught in a surveillance video inside an SUV outside Casino Royale, presumably connected to the attack, said Assistant Attorney General Jose Cuitlahuac Salinas.
The agent is in custody of federal investigators, Salinas told a news conference late Thursday.
Five alleged members of the Zetas drug cartel were arrested Sunday in connection with last week's fire in Monterrey, called one of the worst attacks in Mexico's five-year drug war. Police have said at least 12 people were involved.
Agents are investigating whether the attack was in retaliation for not paying extortion money.
Gunmen last week ignited gasoline at the entrance to the Casino Royale. Terrified customers and employees ran deep inside the building. Many were found dead from smoke inhalation in offices and bathrooms.
Also Thursday, the brother of Monterrey Mayor Fernando Larrazabal was detained for questioning as part of a corruption investigation launched after he was seen taking wads of cash inside an unidentified casino just days before last week's deadly arson attack.
The daily newspaper Reforma, which published the images of Jonas Larrazabal on Wednesday, estimated one wad of cash, stuffed inside a cell phone box, was 400,000 pesos ($32,000).
Jonas Larrazabal's lawyer, Jesus Martinez, told Milenio TV that his client goes to casinos for fun, like any customer, but also sells them cheese, mezcal and other products from the southern state of Oaxaca. He said he has asked his client to provide documents to prove the sales so he can give them to authorities.