While there were no indications that any water supply had been altered or made unsafe to drink, state and local officials throughout the country were on high alert for potential problems in vulnerable computers that are commonly used to monitor and adjust water quality, including chemical-treatment levels and water pressure. Minnesota first publicly reported the attacks, and now Michigan says its systems have also been targeted.

The attack has little precedent, experts said, but has long been the stuff of nightmares and sensationalized Hollywood thrillers: an apparent cyberattack by a foreign power during a time of war that could, at least in theory, jeopardize the health and safety of Americans.

Officials cautioned that they had not definitively determined that Iran was responsible for the attack and that the investigation was still preliminary. But they said Iran had stepped up cyberattacks since the U.S. and Israel launched a war against the country five months ago, and had previously targeted similar water systems and other critical infrastructure in the U.S. There seemed no financial motive, making a criminal attack less likely.