US environmental authorities have found no evidence of cross-border contamination from a toxic mine spill in northwestern Mexico, an official said Friday.

Checks were ordered last month after a massive acid leak in the Sonora River from the Buenavista copper mine, the worst environmental disaster on record in Mexico's mining industry.

The spill turned a 60-kilometer (40-mile) stretch of the waterway orange and caused Mexican authorities to shut off the municipal water supply to 20,000 people in seven towns.

The Arizona Department of Environmental Quality said Friday that "based on tests for presence of eight metals and two chemical compounds," it "found no indication of impact of a reported release from a holding pond at Buenavista Mine in Cananea, Sonora last month."

The International Boundary and Water Commission (IBWC) said at the time that experts had not found any visual signs of pollution.

The San Pedro River, a tributary of the Sonora, flows north from Mexico, entering the United States to the west of Naco, Arizona. The Buenavista mine is located approximately 48 kilometers (30 miles) upstream from the US-Mexico border.

The IBWC is responsible for applying the boundary and water treaties between the two countries.