Washington — The U.S. charged seven hackers linked to the Iranian government with executing large-scale coordinated cyberattacks on dozens of banks as well as a small dam outside New York City — intrusions that law enforcement officials said reached into America’s infrastructure, disrupted the nation’s financial system and cost tens of millions.

Indictments announced Thursday by the Justice Department show a determination by overseas hackers to cripple vital American interests, officials said, and marked the first time the FBI attributed a breach of a U.S. computer system that controls critical infrastructure to a hacker linked to a foreign government.

The hackers are accused of infecting thousands of people’s computers with malware to create a network of zombie computers they used to overwhelm servers of major institutions to knock them offline. Those included the Bank of America, NASDAQ and the New York Stock Exchange.

“The attacks were relentless, systematic and widespread,” Attorney General Loretta Lynch said. “They threatened our economic well-being and our ability to compete fairly in the global marketplace, both of which are directly linked to our national security.”

One of the alleged hackers is accused of repeatedly gaining access to the control system of the Bowman Avenue Dam, a small flood-control structure in Rye Brook, about 20 miles north of New York City. Officials termed his access “a frightening frontier on cybercrime,” and said the hacker would have been able to operate a digitally controlled sluice gate, flooding portions of the city of Rye, but the gate had been disconnected for maintenance.

The hacker was still able to gain information about the dam’s operations, including its water level, temperature and the sluice gate.

While that attack did no harm, one official said the hacker obtained knowledge about the computer system that could be used on other dams and infrastructure. The official spoke on condition of anonymity because he wasn’t authorized to speak publicly.

The indictments unsealed Thursday stem from intrusions between 2011 and 2013 that officials say targeted 46 victims, disabling bank websites and interfering with customers’ ability to do online banking.

The attacks, which occurred sporadically over 176 days, cost the institutions tens of millions of dollars in remediation costs, but no customers lost money or had their personal information stolen.