Federal police arrested a state police officer Thursday 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 Mexico's Attorney General's regional chief Jose Cuitlahuac Salinas.
The agent is in custody of federal investigators, Salinas said in a news conference late Thursday.
Five alleged members of the Zetas drug cartel were arrested Sunday in connection with last week's fire, called one of the worst attacks in Mexico's five-year drug war. Police have said at least 12 people were involved.
Agents investigate 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.