US authorities went as far as enlisting the Vatican to intervene in the case of missing former-FBI agent Robert Levinson, who disappeared in 2007 while allegedly on an unapproved CIA mission in Iran.
A WikiLeaks cable from October 2008 revealed that the Iranian government "chastised" the Holy See Nuncio in Tehran over the affair:
Iran said "it does not have any information about the whereabouts of US citizen Robert Levinson".
The Holy See "has no business expressing interest in the case" Iran added.
Levinson, who disappeared during a business trip to the Iranian island of Kish in March 2007, was allegedly in Iran on a rogue mission with the CIA.
Levinson's family was paid $2.5m (£1.5m) to avoid a public lawsuit. The analysts who hired Levinson - who had no authority to run spy operations - were forced out of the organisation.
The Associated Press said it confirmed ties between Levinson and the CIA in 2010, and revealed the Vatican was enlisted by the State Department, among the other countries enlisted to put pressure on Tehran.
It is still unclear whether the Holy See knew about the alleged intelligence mission.
Iran, which has had diplomatic relations with the Vatican for 53 years, has a strong diplomatic corps in Rome. Ex-pope Benedict XVI was seen as an ace in the hole in strategies to avoid a potential military intervention by Israel or the US.
"Diplomatic relations between the Holy See and the Islamic Republic of Iran are developing and consolidating in every field," said the Iranian ambassador Ali Akbar Naseri in 2011.
The US asked Iran for assistance with the case back in August, following the election of moderate cleric Hassan Rouhani.
But Iranian authorities said Levinson's whereabouts are unknown.
In 2011, Levinson's wife Christine received photos that the US hoped signalled Levinson's abductors wanted to negotiate. Levinson was snapped holding up five signs which read: "Why you can not help me?" and "I am here in Guantanamo - Do you know where it is?"
However, the US government has not received any evidence he is still alive for almost three years, AP reported.
Levinson's family say the US government had not done enough to rescue him. The FBI, for its part, offered a $1m reward for information leading to Levinson's safe return.