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Report Submitted To Prosecutors
The drinking water regulator confirmed a report had been sent to the procurator fiscal in connection with the contamination of drinking water in North Lanarkshire in June last year.
Customers were told not to cook, drink or wash with their tap water following concerns over its taste and smell.
Nine schools were also closed as a precautionary measure as Scottish Water investigated the problem.
The affected properties were in parts of Carfin, Newarthill, Chapelhall, New Stevenston and Dalziel Park.
In her annual report, Sue Petch, Drinking Water Quality Regulator for Scotland, said: "One major incident affecting drinking water quality occurred during June 2015 when over 6,000 properties in North Lanarkshire suffered restrictions on the use of their water for two days after contamination of the supply.
"We have investigated this event thoroughly and submitted a report to the procurator fiscal."
A spokesman for Scottish Water said: "This matter is now with the procurator fiscal and we cannot comment further. Providing safe, wholesome drinking water to our five million customers is Scottish Water's top priority."
The report showed the company's compliance with drinking water standards reached a record high in 2015.
Out of 308,356 samples of water taken from consumers' taps, 99.92% met the required standards.
Of the 35 incidents that were investigated, only the one in North Lanarkshire was classified as major.
Ms Petch said: "Consumers in Scotland are generally more satisfied with the taste and appearance of their tap water.
"Last year, only 0.2% of consumers reported concerns with the quality of their supply, almost half that of numbers reported six years ago.
"When concerns are reported, they are thoroughly investigated and, where necessary, action is taken to resolve the situation.
"It is vital that the progress that has been made is maintained and further built upon.
"Scottish Water must ensure their assets and procedures are resilient and able to cope with a wide range of challenges."
A spokesman for the Crown Office and Procurator Fiscal Service said: "I can confirm that we have a report from the Drinking Water Quality Regulator and that investigations are ongoing."