Iran announced that has released four prisoners, one of whom is reportedly Jason Rezaian, a correspondent for the Washington Post who had been detained since 2014. All four of the prisoners, Iranian-American dual nationals, were reportedly released "within the framework of exchanging prisoners," ISNA news agency quoted the Tehran prosecutor's office as saying, according to the Associated Press.

Rezaian has been in custody in Iran since July 22, 2014. He was convicted of espionage against the Iranian state and "propaganda against the establishment" in October 2015 and then sentenced to a undisclosed prison term in November 2015.

Iran is in possession of three other U.S. prisoners, reported BBC News. While it is unclear who is being released, they are:

• Former U.S. Marine Amir Hekmati of Flint, Mich. He was detained in August 2011 on espionage charges while in the country to visit his grandmother. Though initially sentenced to death in 2012, he got a reduced sentence of 10 years in 2014 following a retrial. His family fears he may contract tuberculosis, citing his significant weight loss and breathing problems that he has experienced since his incarceration.

• Pastor Saeed Abedini of Boise, Idaho. He was detained for compromising national security, likely because of Christian proselytizing, in September 2012, a charge which he had already been arrested for previously in 2009. He was sentenced in 2013 to eight years in prison. There are claims that he was subjected to physical violence during his incarceration..

• Siamak Namazi, head of strategic planning at UAE-based Crescent Petroleum Co. and the son of a politician from the shah's era.

News of the release comes following a tense period in U.S.-Iran relations. In late December, U.S. officials alleged that the Iranian Navy unnecessarily provoked them by launching a test missile nearby a U.S. warship, and early this week, Iran released 10 U.S. naval officers who they held in captivity for a brief moment after a mechanical error caused their two ships to drift into Iranian territorial waters.

The release also comes on the same day of the historic nuclear accord that Iran struck with the U.S. and other world powers was due to be implemented.