The Embassy in Tehran is seeking clarity on how the unidentified prisoner died as the media speculated that he committed suicide.
Switzerland has confirmed the death of one of its citizens who was arrested on espionage charges in the Semnan region of Iran.
Pierre-Alain Eltschinger, spokesman for the Swiss Federal Department of Foreign Affairs, confirmed the death on Thursday, the Associated Press reported. He added that the Swiss embassy in Tehran had contacted the authorities to explain what had happened.
Semnan prison is located about 180 kilometers (112 miles) east of Iran's capital, Tehran.
Iran's Mizan Prosecutor's Office said the man, who has not been identified, asked a friend in prison to bring him food and then killed himself alone.
“Efforts to save him were unsuccessful,” Mizan quoted Mohammad Sadegh Akbari, head of the province's justice department, as saying.
Switzerland plays an important role between Washington and Tehran, representing the interests of the United States in Iran since the 1979 US Embassy seizure and hostage crisis.
Detention of aliens
In recent years, the Iranian military has arrested many foreign nationals and dual nationals, mostly on espionage and security related charges.
Human rights organizations have criticized Iran for trying to disarm other countries through such arrests. Iran strongly denies this.
On Wednesday, an Italian journalist Cecilia Hall, a reporter for Il Foglio daily, was released by the Iranian authorities after three weeks in prison and returned home.
Sala was in the country on a permanent journalist's visa when he was arrested in Tehran last month, accused of “violating the laws of the Islamic Republic”.
Iran has denied claims that his arrest is related to Rome's arrest of an Iranian businessman accused by the United States of complicity in its war.