The small, picturesque town of Rolle, on the shores of Lake Geneva, acknowledged last week that it had been the victim of ransomware attack, and that data on some administrative servers had been compromised.

But the municipal government in the town of some 5,400 inhabitants insisted that only small amounts of data had been affected, and that all the information had been restored from backup copies.

The town administrative chief Monique Choulat Pugnale told the Swiss daily 24 heures at the time that it was "a weak attack", which only affected email servers that "did not contain any sensitive municipal data".

But according to an investigation published by the Le Temps daily Wednesday, the attack first detected on May 30 had in fact been "massive".

The paper cited an unnamed cybersecurity expert on the "dark web" saying it had taken him only 30 minutes to turn up thousands of Rolle municipal documents.

The documents, he said "are personal and extraordinarily sensitive."

Rolle municipality, which has filed a criminal complaint in the case, acknowledged in a statement late Wednesday that it "underestimated the severity of the attack (and) the potential uses of the data.

The town said it "admits with humility a certain naivete towards the stakes when dealing with the dark web and malicious hacks", and said it had set up a taskforce to deal with the crisis.

It did not go into details about the kinds of information that had been exposed, but Le Temps said its journalists had seen spreadsheets with data on all inhabitants.

That data included names, addresses, dates of birth, social security numbers and residency permit information for non-Swiss nationals. In some cases, religious affiliation was also listed.

School records had also been found, with list of students' grades, as well as information on children who had contracted Covid-19.

Forms used to evaluate the performance of communal employees were also found, as were some criminal records, according to the paper.