Story highlights A state prosecutor says inmates were fighting over control of the Topo Chico prison

Violence shows there's "an urgent need to do something" in Mexico's prisons, expert says

Officials say 49 people were killed and 12 injured in an overnight riot at the facility

(CNN) What started as a late-night fight between leaders of rival gangs spiraled into a massive prison riot in northern Mexico that left dozens dead, officials said.

The riot killed 49 people inside Topo Chico prison in Monterrey, a spokesman for the state's governor told reporters Thursday afternoon. Authorities initially announced a higher death toll, but later corrected that figure.

Twelve others were injured, including five who are in critical condition, he said.

Video showed flames rising from the prison -- the result, Nuevo Leon state Gov. Jaime Rodriguez said, of inmates setting fires in two areas inside.

The violent clashes come just one day before Pope Francis arrives in Mexico, a trip that will include a stop at a prison in Ciudad Juarez, a city on the mend after once being known as the murder capital of the world.

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