A US judge has ruled that Tehran was responsible for snatching and torturing a former FBI agent who went missing 13 years ago during an investigation into skimming of Iranian oil profits. The ruling, on the anniversary of Robert Levinson’s disappearance, clears the way for the family’s $1.5 billion claim against the regime. Mr Levinson, 71, was last known to be alive in 2011 because of photographs emailed by his captors. The private investigator travelled to the Iranian resort of Kish Island to question a contact over claims that Iranian officials were taking a cut from oil sales and hiding the money in overseas investment. He checked out of his hotel following the meeting but has not been seen since March 9, 2007. A video and photos received by his family suggested he was being held by an unidentified terrorist group but judge Timothy Kelly ruled that Iran was responsible. “If he is alive, he would be the longest-held civilian hostage in American history,” said the judge. The case brought by Mr Levinson’s wife and his seven children “is largely about whether it was the Iranian regime that committed these barbarous acts”, said Judge Kelly. “The court finds, in no uncertain terms, that it was.” The court had heard in December that Iran’s Islamic Revolutionary Guard Corps was probably behind the imprisonment of Mr Levinson. His investigation into state corruption was a reason to detain him, the judge said. The judge said Iranian involvement was further suggested by a story in state-run Press TV less than a month after he was snatched. The report said the unidentified US businessman had been “in the hands of Iranian security forces since the early hours of March 9”. The court also heard that Iran’s ambassador to Paris summoned a go-between in 2011 and tried to broker a deal that would see his release in return for the delay of a report from the International Atomic Energy Agency that was likely to be critical of Tehran. The ruling comes four months after Christine Levinson, the missing man’s wife, and his seven children gave evidence detailing their anguish about the continued absence of the investigator and the impact it had on their lives. On their wedding days, his daughters tied his picture to their bouquets so they could say their father walked them down the aisle, the judge said. Iran has not responded to the lawsuit. The judge found that “despite Iran’s apparent denials of responsibility for Levinson’s abduction, there are no other plausible explanations in the record for what happened to him”. The damages – which will be decided at a separate hearing – are payable from a fund made up of seized assets and fines levied against banks for breaching Iranian sanctions. US courts have made awards of $46bn to victims of Iranian-backed terrorism including the families of 241 US soldiers killed in the 1983 bombing of a US marine barracks in Beirut. Mr Levinson’s family said that his disappearance was the “beginning a nightmare for him and for our family that continues to this day”. On this anniversary, the United States District Court for the District of Columbia has found that the Iranian regime is responsible for what happened to him,” they said in a statement. “We will continue to do everything in our power to seek justice for our husband and father.”