MONTERREY, Mexico - Vast luxuries such as saunas, a bar, food stands and appliances were discovered by authorities entering a prison in northern Mexico to investigate a riot that killed 49 inmates.

Nuevo Leon state authorities said in a statement late Sunday that the cells inside Monterrey's Topo Chico prison were outfitted with mini fridges, air conditioners, digital cable and aquariums. There were 280 food and grocery stands where inmates could buy goods.

The riot broke out Thursday between two rival factions of the Zetas drug cartel. It has since opened searing questions about gang rule, extortion and human rights violations in Mexico's overcrowded prisons, where people merely awaiting trial are mixed in with some of the world's most hardened killers.

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Heavy machinery was brought in to haul away tons of contraband furniture and other goods that authorities had piled in the prison yard, the statement said.

It added that police destroyed hundreds of altars to the Death Saint - some of them life-size. The folk figure is revered by drug traffickers and some people among the downtrodden.

"We knew about all of the irregularities that existed, arbitrary acts, abuses, taxes," Gen. Cuauhtemoc Antunez, the state's security secretary, said in the statement. Authorities did not say how long the abuses had occurred or who allowed them.

Nuevo Leon Gov. Jaime Rodriguez said last week that 60 hammers, 86 knives and 120 shivs were used in the melee.

The prison's director, superintendent and a guard have been arrested on murder charges.

On Monday, a United Nations official called on Mexico's government to conduct an exhaustive investigation of a prison riot that killed 49 last week.

Special rapporteur for torture Juan Mendez said in statement that the government must guarantee that the victims' families know what happened during the Thursday riot at the Topo Chico prison in the northern city of Monterrey.

The investigation should identify those responsible and include reparations for the victims' relatives, Mendez said.

The U.N. official toured the prison in 2014. He said he witnessed horrible conditions and lax rules that allowed prisoners to govern themselves and led to violence.

The riot at Topo Chicago began around midnight with prisoners setting fire to a storage area, sending flames and smoke billowing into the sky. Rescue workers were seen carrying injured inmates - some with burns - from the facility.

Nuevo Leon state Gov. Jaime Rodriguez said the clash was between two factions led by a member of the infamous Zetas drug cartel, Juan Pedro Zaldivar Farias, also known as "Z-27," and Jorge Ivan Hernandez Cantu, who has been identified by Mexican media as a Gulf cartel figure.

But National Security Commissioner Renato Sales Heredia said later Thursday in a radio interview that authorities believe the fight was between two factions of the Zetas for control of the prison.

A turf war between the gangs bloodied Nuevo Leon state and neighboring Tamaulipas between 2010 and 2012. The Zetas once nearly controlled the area around Monterrey.

The situation at the prison was so out of control that even Rodriguez acknowledged to local media that the two cartel bosses "were fighting for control" of the prison.