The death of 3.2 million fish at a lagoon in western Mexico this past week was caused by poorly functioning wastewater treatment plants that failed to filter out untreated material, authorities said.

An analysis of water samples taken at the Cajititlan lagoon in Jalisco state found that at least 82 tonnes of fish turned up dead because they lacked oxygen due to excessive organic waste in the water.

"We are talking about poor functioning of the treatment plants in the municipality," Jalisco environment secretary Magdalena Ruiz said late Tuesday, citing the study by the state's water commission and forensic institute.

She said "raw and untreated waste" was released into the lagoon, killing more than 3.2 million fresh water fish known as "popocha." The analysis examined water samples taken there since October.

It was the fourth mass fish death at the same lagoon this year.

The municipality of Tlajomulco had blamed the deaths on cyclical changes of temperature that reduce the amount of oxygen in the water.

State prosecutors have launched an investigation.

In July 2013, some 500 tonnes of fish died in a Jalisco reservoir after a company that made food for livestock without a permit dumped huge amounts of molasses into the water.