Four months after a historic accord with Tehran to limit its atomic ambitions, American officials and private security groups say they see a surge in sophisticated computer espionage by Iran, culminating in a series of cyberattacks against State Department officials over the past month.

The surge has led American officials to a stark conclusion: For Iran, cyberespionage — and the power it gives the Iranians to jab at the United States and its neighbors without provoking a military response — is becoming a tool to obtain the influence that many in Iran hoped the nuclear program would give the country…

“It was very carefully designed and showed the degree to which they understood which of our staff was working on Iran issues now that the nuclear deal is done,” said one senior American official who oversees much of that operation and who requested anonymity to discuss a continuing investigation. “It was subtle.”