Iran hacked the emails and social media accounts of State Department officials during a crucial stage of the nuclear deal in 2015, according to multiple sources familiar with the details, The Washington Free Beacon reported on Wednesday.

Critics of the nuclear agreement said the Obama administration did not publicly talk about the cyberattack, because they feared it could harm support for the arrangement, especially at such a sensitive time.

The attack took place just after the deal barely overcame opposition in Congress in September 2015, and as Iranian and international officials were beginning the process of concluding what critics described as "secret side deals."

Hudson Institute senior Middle East analyst Michael Pregent spelled out the dilemma to the Free Beacon.

"Within hours of the Iran deal being greenlighted, Iran was already conducting cyber attacks against the very State Department that ensured passage of the [deal]," he said, stressing that "acknowledging a cyberattack after the [deal] was greenlighted would be something that would immediately signal that it is a bad deal — that these are nefarious actors."

Asked about the cyberattack, a State Department spokesman said, "For security reasons we cannot confirm whether any hacking incident took place."

But an email dated Sept. 24, 2015 shows that at least four State Department officials were working on trying to contain the hack.

Although there had been much cyber warfare between Iran and the United States over the years, this particular hack was considered unusual, because there had been a limited détente while the nuclear negotiations progressed in 2015.

Hacking into State Department emails at that time would have given the Iranians an important negotiating advantage by knowing the U.S. position on a number of sensitive issues, according to former U.N. weapons inspector David Albright, who is Institute for Science and International Security president.