(CNN) -- At least seven inmates were killed and 20 were injured Thursday in a prison riot in the Mexican border state of Nuevo Leon, authorities said.
Four of the inmates burned to death after inmates set mattresses on fire during the riot, which began around 8 a.m., firefighter Guillermo de Leon Delgado said.
It took firefighters nearly an hour to control the blaze, he said.
Three inmates died of stab wounds, state spokesman Jorge Domene told reporters.
The riot occurred in the Cadereyta prison, located in the Monterrey metropolitan area.
Authorities were investigating the killings, Domene said.
He did not provide details about what caused the fighting. State government officials said more information would be released soon.
In May, 14 inmates died in a fire at a prison in the nearby municipality of Apodaca. Dozens more were injured.
CNNMexico.com's Rodrigo Aguiar and journalist Victor Badillo contributed to this report.