MEXICO CITY—In what Mexican officials have called an "act of terror," a half dozen gunmen burst into a casino in Mexico's industrial capital of Monterrey Thursday, sprayed the place with gasoline, and started a fire in the bingo section, killing at least 53 people.

Speaking to Monterrey's leading newspaper El Norte, Nuevo León Gov. Rodrigo Medina said that at least 53 people had been killed in the attack, making it the bloodiest attack suffered by this prosperous city, which has been rocked by spiraling drug violence in the...