A surveillance camera at Juarez's Cereso prison captured armed men and inmates firing automatic weapons at a holding cell Monday night.
Investigators in Juarez said that at least 17 inmates were found dead after what was originally described as chaotic prison riot. Investigators also said they found 729 shell casings.
The video shows two men wearing white hoodies holding a prison guard against the wall. A second guard leaning against a counter calmly watches the scene unfold. There's an exchange of what appears to be the keys to the holding cell before four other men walked in with automatic weapons.
As the door to the holding cell is being opened, three prison guards can be seen leaving the room. That's when two of the gunmen charge inside and open fire inside the cell.
The gunmen are also seen reloading their weapons and re-entering the cell.
It takes the gunmen about one minute and 40 seconds from the time they walk in until the gunfire stops and the they walk away.
After the killings, Chihuahua state police say some inmates tried to escape but were caught several in tunnels. Authorities are trying to determine who let the weapons inside the building.
Investigators said that four young women were inside the prison for a "party" in which 40 inmates participated.
Officials said that 400 inmates have been tested for traces of gunpowder and out of the six weapons used, an AR-15 rifle has been located inside the prison.
Forty-two guards and 116 inmates have been interviewed.