The Daily Mail reports the accident occurred as a tank was undergoing maintenance cleaning. According to plant manager Francisco Lopez Bravo, the workers were trapped inside the cisterns while cleaning.
City prosecutors are currently investigating whether inhaling toxic fumes caused the employees' deaths.
A Grupo Modelo representative issued a statement apologizing and offering compensation to those affected by the tragedy.
"Modelo is deeply sorry for this incident and will support the affected families permanently."
According to Lopez Bravo, the plant resumed operations Sunday as there were no further risks. Grupo Modelo's Mexico City Plant was the company's fist. It began operating in 1925.