Italian authorities have opened an investigation into a "very serious" attack on the IT system of the health department of Lazio, one of the most populated regions of Italy, which includes the city of Rome that targeted an official website for booking COVID-19 vaccinations.

The website was unavailable for several hours on Sunday preventing citizens from booking appointments for a coronavirus vaccine, as well as other services, according to a statement posted to the official Lazio government Facebook page.

Nicola Zingaretti, president of the Lazio Region, said that the initial attack took place on Saturday night into Sunday morning and that it "blocked almost all of the files in the data center."

While the government officials did not say who was behind the incident, the local media reported that cyberattack appears to have been launched by hackers affiliated with a ransomware group.

The attackers reportedly breached computer systems of LAZIOCrea, a third-party company in charge of scheduling and booking vaccination appointments, and from there they were able to deploy malware, which encrypted data on the system, including data associated with a large public health database.

Local authorities said they have received a generic ransom request but with no further claims.

Zingaretti said that 70% of adult population in Lazio have been vaccinated, despite the cyberattack.