Iran is acknowledging for the first time it has an open case before its Revolutionary Court over the 2007 disappearance of a former FBI agent on an unauthorized CIA mission to the country.

In a filing to the United Nations, Iran said the case over Robert Levinson was 'on going,' without elaborating, but the development has renewed questions over what happened to him.

It wasn't immediately clear how long the case had been open, nor the circumstances by which it started.

However, it comes amid a renewed push to find him with an offer of $20 million for information from the Trump administration amid heightened tensions between Iran and the U.S. over Tehran's collapsing nuclear deal with world powers.

Iran has admitted for the first time it has an open case against former FBI agent Robert Levinson. He is pictured in one of a series of images obtained by Radio Free Europe/Radio Liberty group which handed them over to Levinson's wife Christine in April 2011, who opted not to release them at that time. It is not clear how Radio Liberty came into possession of the photos, taken after Levison was taken hostage

An FBI poster showing a composite image of former FBI agent Robert Levinson, (right), of how he would look like now after five years in captivity, and an image, (center), taken from the video, released by his kidnappers, and a picture before he was kidnapped, (left)

The money is in addition to $5 million that had been earlier offered by the FBI.

The Associated Press on Saturday obtained the text of Iran's filing to the U.N.'s Working Group on Enforced or Involuntary Disappearances.

'According to the last statement of Tehran's Justice Department, Mr. Robert Alan Levinson has an on going case in the Public Prosecution and Revolutionary Court of Tehran,' the filing said.

Iran's Revolutionary Court typically handles espionage cases and others involving smuggling, blasphemy and attempts to overthrow its Islamic government.

Westerners and Iranian dual nationals with ties to the West often find themselves tried and convicted in closed-door trials in these courts, only later to be used as bargaining chips in negotiations.

Iran's mission to the U.N. did not immediately respond to a request for comment, and its state media has not acknowledged the case. The U.S. State Department did not respond to a request for comment about Iran's acknowledgement.

The Washington Post first reported on the ongoing case. Levinson disappeared from Iran's Kish Island on March 9, 2007.

For years, U.S. officials would only say that Levinson, a meticulous FBI investigator credited with busting Russian and Italian mobsters, was working for a private firm on his trip.

In December 2013, the AP revealed Levinson in fact had been on a mission for CIA analysts who had no authority to run spy operations.

Levinson's family had received a $2.5 million annuity from the CIA in order to stop a lawsuit revealing details of his work, while the agency forced out three veteran analysts and disciplined seven others.

He was pictured in a series of images obtained by Radio Free Europe/Radio Liberty group which handed them over to Levinson's wife Christine in April 2011, who opted not to release them at that time. One of the photos show him wearing an orange jumpsuit and chains holding a sign that read: 'This is the result of 30 years serving for USA.'

It is not clear how Radio Liberty came into possession of the photos, taken after Levison was taken hostage.

Levinson (center with what appears to be family) disappeared from Iran's Kish Island on March 9, 2007. For years, U.S. officials would only say that Levinson, a meticulous FBI investigator credited with busting Russian and Italian mobsters, was working for a private firm on his trip

Robert Levinson's family said they are encouraged and claimed the Trump administration is 'completely committed' to bring him home

Since his disappearance, the only photos and video of Levinson emerged in 2010 and 2011.

He appeared gaunt and bearded with long hair, and was wearing an orange jumpsuit similar to those worn by detainees at the U.S. prison at Guantanamo Bay.

The video, with a Pashtun wedding song popular in Afghanistan playing in the background, showed Levinson complaining of poor health.

Rumors about him have circulated for years, with one account claiming he was locked up in a Tehran prison run by Iran's paramilitary Revolutionary Guard and U.S. officials suggesting he may not be in Iran at all.

Dawud Salahuddin, an American fugitive living in Iran who is wanted for the assassination of a former Iranian diplomat in Maryland in 1980, is the last known person to have seen Levinson before his disappearance.

Secretary of State Mike Pompeo attended an event earlier this week to mark the 40th anniversary of the Iranian hostage crisis and said the Trump administration is focused on ensuring the safe return of those still detaine

Christine Levinson (center), wife of former FBI agent Robert Levinson, her son Daniel Levinson (left) and her sister Suzan, (right) attend a news conference at Switzerland's embassy in Tehran in December 2007

Iran has offered a series of contradictory statements about Levinson in the time since.

It asked the U.N. group to close its investigation into Levinson in February, saying 'no proof has been presented by the claimant in this case to prove the presence of the aforesaid in Iran's detention centres.'

This week also marks the 40th anniversary of the start of the Iran hostage crisis in which Iranian students stormed the U.S. Embassy in Tehran and took 52 diplomats hostage.

In apparent reference to this, the Levinson family issued a statement earlier this week to The Washington Post saying: 'Bob Levinson has been held more than 10 times longer — for 4,624 days.

Dawud Salahuddin, an American fugitive living in Iran who is wanted for the assassination of a former Iranian diplomat in Maryland in 1980, is the last known person to have seen Levinson before his disappearance

Supporters gather during a rally for Robert Levinson in Florida in 2016. At that time, family members expressed disappointment that he wasn't part of a January prisoner exchange with Tehran

'Bob Levinson must come home, and Iran's hostage-taking as government policy must end.'

The Washington Post had reported that Levinson's family filed a complaint with the United Nations Working Group on Enforced or Involuntary Disappearances in 2016.

At the time, they were told not to believe that Iran would respond as they did not respond to similar cases previously.

But they were informed last month that Iranian authorities had replied by claiming their courts had an 'ongoing case' with him.

Sarah Moriarty, one of Levinson's daughters, told the outlet this week: 'If they have charges against him, what are they?.

Speaking about the latest development she claimed: 'We're encouraged. This administration is engaged and completely committed to bringing my dad home.

'They've taken it as a personal mission. I think they care passionately about bringing my dad and other Americans home.'

Secretary of State Mike Pompeo attended an event earlier this week to mark the 40th anniversary of the Iranian hostage crisis and said the Trump administration is focused on ensuring the safe return of those still detained.

He was reported by The Washington Post as saying: 'The Trump administration has made clear that the regime in Iran must release all missing and wrongfully detained Americans, including Robert Levinson, Xiyue Wang, Siamak Namazi and others. We will not rest until they are reunited with their families.'