U.S. Secretary of State John Kerry called on Iran on Monday to release a former Marine who he said has been “unjustly” held for nearly three years, AFP reports.

The Marine in question is Amir Hekmati, who was arrested in 2011, put on trial and found guilty of spying for the CIA.

He was subsequently sentenced to death, a penalty that was recently overturned and reduced to 10 years in prison.

“Mr. Hekmati has spent almost three years in an Iranian prison on false espionage charges. We remain especially concerned about reports of Mr. Hekmati’s health in prison,” Kerry said in a statement quoted by AFP.

“Hekmati’s family in the United States has endured the hardship of his absence for too long. He is the eldest son, and his family misses him and needs him home,” he added.

“We respectfully request that the government of the Islamic Republic of Iran release Mr. Hekmati so that he may be reunited with his family in the United States,” said Kerry.

Hekmati’s family and the U.S. government say the dual U.S.-Iranian citizen is not a spy and went to Iran to visit his grandmother.

Several months ago, the White House asked Iran to release Hekmati, as well as Saeed Abedini, a Christian pastor who was jailed for his religious beliefs.

The White House also asked Iran for help in locating former FBI agent Robert Levinson, an American citizen who was apparently working as a private detective when he went missing during a business trip to Iran’s Kish Island in March 2007.