A Mexican copper mine which spewed millions of gallons of acid into a river last month is still causing pollution and the facility's owners are blocking the work of investigators probing the accident, authorities said.

The massive acid leak in August, involving some 40,000 cubic meters (10.6 million gallons) of sulfuric acid, was one of Mexico's largest ever mining-related environmental disasters.

"As of this moment, the government of Sonora (state) totally breaks off any relationship with the mining company," which is continuing to discharge toxic substances in the river, director of the state civil protection agency, Carlos Arias said at a press conference Friday.

The toxic acid, used to dissolve copper from ore, spilled out of a holding tank at the Buenavista copper mine in Sonora State, one of the largest in the world.

The chemical turned a 60-kilometer (40-mile) stretch of the Sonora River orange, causing authorities to shut off the municipal water supply to 20,000 people in seven towns.

Arias said since the spill, Buenavista, a subsidiary of Grupo Mexico, has blocked access to investigators, and he warned Sonora state authorities would come back -- this time backed up by security forces.

"We will act with the full weight of the law, because they are already in a plan that cannot continue," Arias said, adding the government was mulling permanent closure of the mine.

The mining company "categorically denied the accusations," in a statement Friday night.

"Buenavista del Cobre has worked alongside state authorities," the company said, lamenting "the politicization of the accident."

The mining company has created a fund of two billion pesos ($147 million) to repair the environmental damage. Environmental authorities have also imposed fines of more than 44 million pesos ($3 million) over the spill.

Federal prosecutors are still investigating whether the leak was caused by shoddy construction and installation of the pipeline or, as the company argues, by excessive rains.