By Euan McKirdy

CNN

(CNN) -- The family of a former FBI agent who disappeared in Iran a decade ago is suing the country, according to court documents obtained by CNN.

The wife and children of Robert Levinson, a former FBI agent whom US investigators believe is being held in Iran, filed a suit with the US District Court in Washington, D.C.

The complaint states that the family is filing suit under the Foreign Sovereign Immunities Act "for injuries suffered by each of them as a result of Iran's unlawful acts of hostage taking, torture and other torts."

If indeed Iran does have him in custody, Levinson would be the longest held US civilian by a foreign government. Iran has denied having anything to do with his disappearance and has said they don't know where he is.

CNN is attempting to reach the Iranian government for response to the lawsuit but most offices are closed for New Year celebrations.

Claims of mistreatment

"Three weeks after his disappearance an Iranian government news outlet, Press TV, announced that he had been taken into custody by Iranian security authorities but was expected to be released shortly," the complaint alleges.

"Despite this report, Robert Levinson was not released and the Iranian government began falsely denying any knowledge of his capture or whereabouts."

It also alleges that the Iranian government had falsely concocted a ruse involving several emails containing a proof-of-life video and photos of Levinson in a disheveled state, sent to the Levinson family, making it appear that a terrorist organization had kidnapped the American.

"The threatening messages, along with... video and photographs... were enormously disturbing to the Levinson family," it says. "They were intended to and did create significant emotional trauma in the entire family."

It also alleges that the family is aware that "Iran routinely uses physical and psychological torture in the questioning of its prisoners.

"Given this pattern, Robert Levinson's circumstances and his appearance and actions in the hostage-related videos and photographs... it is reasonable to believe that Iran has used various forms of physical and psychological torture on Levinson."

Ten year disappearance

According to State Department officials, Levinson traveled to Kish Island in Iran in March 2007.

Reportedly, Levinson was in the Mideast to investigate cigarette smuggling on behalf of a client and during the visit, met with American fugitive Dawud Salahuddin, who remains the last person to acknowledge seeing him on the day of his disappearance. Salahuddin told TIME magazine that Iranian security officials detained the two, but while he was released the following day, Levinson was never seen again.

After working for the DEA and then FBI, Levinson was hired as a contractor by the CIA, to write reports for the agency, in 2006.

Despite government insistence that he was in Iran as a private contractor, several media outlets, including the New York Times and the Washington Post, reported in 2013 that Levinson was in Iran on behalf of the CIA, and was attempting to turn Salahuddin into an intelligence asset. The White House said in response that Levinson was not a government employee at the time of his disappearance.

Earlier this month, on the tenth anniversary of his disappearance, Levinson's family released a statement lambasting Iran for the alleged detention, and the US authorities for their inaction in attempting to repatriate him.

"For 10 years the government of Iran has been allowed to dodge and weave every time it was asked to come clean about what happened to Bob and send him home," the statement, released March 9, reads.

"Where is the outrage of this treatment of an American citizen? For 10 years, over and over and over again, two US Presidents abandoned him, a lifelong public servant."

Investigators: Levinson may still be in Iran

FBI investigators believe Robert Levinson, if he is still alive, is being held in Iran despite some US officials saying the proof-of-life video and photos sent to his family in 2010 and 2011 gives clues that indicate Levinson was in Pakistan or elsewhere.

At the time, an analysis of the video suggested as much. What is in agreement among US agencies is the possibility that Levinson died years ago in captivity, the officials say.

Shortly after releasing the ten-year anniversary statement, Levinson's children told CNN they believe US President Donald Trump will be able to bring their father home.

"We are very confident that President Trump has the deal-making skills that are necessary in order to bring him home and to take a stronger stance with Iran and demand his release," Levinson's daughter, Stephanie Curry, told CNN's Alisyn Camerota on "New Day."

His son, David, said he believes Trump will keep their father at the forefront of any future negotiations with Iran.

During his career at the FBI, Levinson specialized in investigating organized crime in Russia.