Sixteen inmates were killed and five were wounded in a prison fight in the northern Mexican state of Zacatecas, authorities said on Wednesday, in one of the worst outbreaks of violence in the country's troubled penal system since President Andres Manuel Lopez Obrador took power.

The fight broke out on Tuesday afternoon at the Regional Center for Social Reintegration in the town of Cieneguillas, located on the western flank of state capital Zacatecas, the state government said in a statement issued on Tuesday evening.

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An inmate was arrested with a firearm and three more guns were found in the prison, as well as various knives.

The government has launched an investigation to determine those responsible for the fight and how the weapons entered the prison, according to the statement.

Security will be boosted at prisons in the state after the fight, the government said.

The incident marks the latest blow to the security track record of Lopez Obrador, who took power in December 2018 pledging to reduce record levels of violence. Instead, Mexico was in 2019 on track to surpass the previous year’s total of homicides, according to latest data.

Mexican prisons have long been plagued by violence. In 2017, at least 28 inmates were killed when a brutal fight broke out in a prison in the Mexican Pacific resort of Acapulco.