Since the coronavirus outbreak began in Italy in late February, the public has been relying on official government data for a picture of how many deaths, cases, and recoveries there have been in each part of the country, and how things are changing.

READ ALSO: How Italy has changed the way it reports the daily coronavirus figures

Italy's Department for Civil Protection releases these statistics daily at around 6pm. The data is then pored over by experts worldwide, looking for evidence that Italy's quarantine measures are containing the spread as hoped – and that other countries can confidently follow its example.

But a growing number of Italian experts have now said these numbers may not be completely reliable.

“For sure, the figures are wrong,” said Matteo Villa, a researcher at the Italian Institute for Political Studies and author of a new study (in Italian) titled ”Coronavirus: Lethality in Italy, between appearance and reality”

He that the death toll may have been underestimated by up to 6,000, or a third of the official total.

In total, Italian authorities have recorded 115,242 cases and 13,915 deaths as of Thursday, April 2nd.

Authorities acknowledge that the data are incomplete, as the death toll doesn't include people who died at home, or in nursing homes, or those who were infected by the virus but not tested.

“It is plausible that deaths are underestimated,” Higher Health Institute president Silvio Brusaferro stated. “We report deaths that are signalled with a positive swab. Many other deaths are not tested with a swab.”

The possible scale of the disparity showed up this week when death records for the month of March from different years were compared.

The data showed a very large difference between the number of Covid-19 deaths reported by authorites, and the total death toll in different Italian regions.

Experts have said the only explanation is that coronavirus deaths have been under-reported.

The deaths were “mostly attributable to the virus”, said Villa.

“Some of the increase will be people who die of other illnesses, because they cannot find a hospital bed due to the crisis,” Villa explained, “but that will be partly offset by the decline in road fatalities due to Italy’s lockdown.”

“For every two sufferers in Italy there is another one you can’t see,” he added.

Even higher numbers in Bergamo

Records released by town halls in Bergamo, at the heart of the outbreak in the worst-affected Lombardy region, show a number of uncounted virus deaths which may be even higher than Villa’s estimate.

Bergamo Mayor Giorgio Gori said on Wednesday he does not trust the official figures and thinks the real toll may be twice as high.

The mayor tweeted a newspaper analysis suggesting that the COVID-19 toll in the Bergamo province was “between 4,500 and 5,000, and not the 2,060” officially reported.

The explanation may be that Italy’s count of 115,242 cases is also incomplete, with estimates of the real number ranging from 400,000 to six million.

The mayor also cited analysis showing that 26 percent of Bergamo province's population may have contracted the virus.

Why is Italy's fatality rate so high?

From the offical figures, it appears that the fatalty rate in Italy is around 12 percent.

“That's a nearly impossible number;” Villa told Al Jazeera in a television interview on Thursday.

“We know from other research that the number of people who die are about one percent of those infected,” he said. “So there's something strange going on. Everyone's asking if Italy is special in any way, in terms of why people are dying so much.”

“The simple answer is, clearly not,” he said. “We are not testing enough people.”

These figures have been overestimated throughout the crisis, according to Nino Cartabellotta, a leading Italian public health expert, professor, and president of the Gruppo Italiano per la Medicina Basata sulle Evidenze (GIMBE), Italy's Group for Evidence-based Medicine.

Analysis by GIMBE of the official data appears to show that the mortality rate in Italy has long been well above average. 

Cartabellotta told The Local this rough figure is “undoubtedly overestimated” as “we're only seeing the tip of the iceberg”.

He calculates that, with further testing, the figure would eventually be found to be closer to 2.2 percent, in line with the rate in China.

The head of Italy's Civil Protection Department, Angelo Borrelli, has meanwhile said it's “credible” to believe there up to ten times more cases in reality than officially recorded.

Villa tweeted on Friday that “the difference between the official and plausible total cases is getting deeper.”

The data on official cases is getting “less and less reliable”, he wrote.