Iran is the main suspect in a series of cyberattacks that affected water supply systems in at least seven U.S. states, The New York Times reports, citing federal officials.

The source of the attacks has not yet been officially established. According to the newspaper, the investigation is ongoing, but U.S. authorities believe that hacker groups linked to Iran may be behind the attacks.

More than 30 water supply systems were targeted, including in the state of Minnesota, where problems were recorded on Sunday and Monday. Authorities in Michigan also reported cyber incidents.

Suspicion toward Iran intensified after U.S. security services had earlier warned about the activities of Iranian hackers against critical infrastructure facilities. In a joint notice, the FBI, the U.S. Cybersecurity and Infrastructure Security Agency (CISA), and other agencies said that groups linked to Iran are targeting water supply and wastewater treatment systems, as well as other strategically important facilities.

Nate George, the mayor of the city of Braham, Minnesota, said that the FBI is “almost certain” that Iranian hackers were behind the attack, but is not yet ready to confirm this publicly.

Wired magazine also reported obtaining an internal document from cybersecurity specialists noting that the attack methods resemble the operations of groups linked to Iran.

Commenting on the cyberattacks in Minnesota, U.S. President Donald Trump said that responsibility for them is tied to the actions of the state authorities, including Democratic Governor Tim Walz.