State security spokesman Roberto Alvarez said 20 to 30 people were holding a ceremony at the spring Sunday, praying for rains and a good harvest. Then three men arrived, at least one of them armed with an assault rifle, and opened fire, he said.
Alvarez said there had been a report a week earlier of a dispute over control of the spring. When the army responded to the shooting, they found two fields of poppies in the area.
The spring is near the community of Plan de Ayala in the municipality of Acatepec.
In northern Mexico, meanwhile, authorities in the border state of Nuevo Leon found foam coolers containing body parts at multiple locations Monday.
State public safety secretary Aldo Fasci said the coolers were left outside two prisons and a police station to make sure they received maximum publicity. It was not immediately clear how many victims were contained in the coolers, he said.
Notes left with the coolers contained threats purportedly from one organized crime group to another.