The arrest in 2011 of Mojtaba Atarod for violating export controls went mostly unnoticed in the US. But his release in April of this year is apparently tied to backchannel negotiations between Iran and the US that have been going on in Oman for years.

Times of Israel:

American and Iranian officials have been meeting secretly in Oman on and off for years, according to a respected Israeli intelligence analyst, Ronen Solomon. And in the past three years as a consequence of those talks, Iran released three American prisoners, all via Oman, and the US responded in kind. Then, most critically, in April, when the back channel was reactivated in advance of the Geneva P5+1 meetings, the US released a fourth Iranian prisoner, high-ranking Iranian scientist Atarodi, who was arrested in California on charges that remain sealed but relate to his attempt to acquire what are known as dual-use technologies, or equipment that could be used for Iran's military-nuclear programs. Iran has not reciprocated for that latest release.

Solomon, an independent intelligence analyst (who in 2009 revealed the crucial role played by German Federal Intelligence Service officer Gerhard Conrad in the negotiations that led to the 2011 Gilad Shalit Israel-Hamas prisoner deal), has been following the US-Iran meetings in Oman for years. Detailing what he termed the "unwritten prisoner exchange deals" agreed over the years in Oman by the US and Iran, Solomon told The Times of Israel that "It's clear what the Iranians got" with the release of top scientist Atarodi in April. "What's unclear is what the US got."

The history of these deals, though, he said, would suggest that in the coming months Iran will release at least one of three US citizens who are currently believed to be in Iranian custody. One of these three is former FBI agent Robert Levinson.

Solomon told The Times of Israel that the interlocutor in the Oman talks is a man named Salem Ben Nasser al Ismaily, who is the executive president of the Omani Center for Investment Promotion and Export Development and a close confidant of the Omani leader Sultan Qaboos bin Said.

The American hikers arrested in Iran in 2009 and released a year later were apparently part of these backchannel prisoner releases. But it isn't clear what other US citizens have been released by Iran.

The release of Atarod was unexpected. The indictment against him is sealed, suggesting national security might be an issue. In other, similar cases involving Iranians, there were attempts to circumvent export controls and purchase "dual use" equipment that could be utilized by the military in construction of a nuclear bomb.

We don't know if that's the case here, but it doesn't matter now that Atarod is back in Iran.