A police officer arrives at a state prison during a riot Wednesday in Ciudad Juarez, Mexico. A fight between gangs after a conjugal visit at a state prison in the Mexican border city of Ciudad Juarez has left at least 20 prisoners dead and seven others injured, police said.

Ciudad Juarez, Mexico ? A fight between gangs at a state prison in the Mexican border city of Ciudad Juarez left at least 20 prisoners dead and seven others injured on Wednesday, police said.

Some gunshot wounds were reported and police were investigating how a firearm entered the prison, said Victor Valencia, the Chihuahua state governor’s representative.

The fight broke out shortly after 6 a.m. as visitors were leaving after an evening of conjugal visits, said Marco Antonio Moreno, state police spokesman in the city across the Rio Grande from El Paso, Texas.

An Azteca gang member “pressured” a guard to open the cells of fellow gang members as the conjugal visit was ending, Valencia said, without describing the pressure.

About 150 members of the gang then barged in to the area where inmates were saying goodbye to their loved ones and attacked members of two other gangs, Valencia said.

About 200 anti-riot police and 50 soldiers regained control of the prison three hours later, said Enrique Torres, spokesman for the joint federal police and military operation in Chihuahua state.

Three of the injured were hospitalized and one was listed in serious condition, Valencia said.

Wednesday’s fight was the worst violence recorded at the four-year-old state prison, which has about 730 inmates and is built to hold 1,000.

Authorities were still assessing the damage, which included broken windows, broken fences and burned mattresses, officials said.