According to Hekmati's family and friends, who live in the United States, he is not a spy and was visiting his grandmother in Iran. His family told CNN they have struggled to hire an Iranian attorney to defend Hekmati.

Tensions between the United States and Iran have heightened over the past month as Iran threatened to close the Strait of Hormuz, a key oil passageway in the Persian Gulf, and the United States imposed increased sanctions on the Iranian central bank.

The White House and State Department have maintained that Tehran is seeking distractions from Iran's domestic problems.

State Department spokesman Mark Toner said last month in a briefing that the United States had called on the Iranian government to release Hekmati and allow the Swiss protecting power access to him in jail. The Iranian government had previously refused the Swiss consular access to the prisoner. 

"We’ve seen this story before, with the Iranian regime falsely accusing people of being spies and then holding innocent foreigners for political reasons," Toner said in December, as Iran reportedly began a closed-door trial.

Hekmati, 28, is a former Marine who was raised in Michigan.