Iranian investigative journalist Hossein Dehbashi has been exonerated in a lawsuit filed against him for saying that the son of the founder of the Islamic Republic died of "drug abuse".

Dehbashi said in a tweet on Wednesday that the complaint by the institute for the works of Ruhollah Khomeini as well as his grandson Hassan for publishing lies and insulting Ahmad Khomeini was rejected.

In February, he tweeted the picture of Ruhollah Khomeini disembarking from a plane in Tehran in February 1979, after a 15-year exile, explaining what happened to those who accompanied him on the flight.

In the post, Dehbashi had said that the reason for Ahmad Khomeini's death was "drug abuse", which led to a barrage of criticism against him.

He later deleted the post but said in reaction to critics that he heard the allegation from Akbar Hashemi Rafsanjani, who was the most influential man in the Islamic Republic, second only to Supreme Leader Ali Khamenei.

Khomeini's son, Ahmad, who also accompanied his father in the return journey and was his confidant until the ayatollah's death, died in a mysterious incident in 1995 that was reported as a heart attack.

Another journalist, Emadeddin Baghi, quoted regime insiders as having said that former deputy intelligence minister Saeed Emami had confessed to Ahmad Khomeini’s killing.

Emami was also accused of having independently organized the "chain murders" -- a series of 1988–98 murders and disappearances of Iranian dissident intellectuals who had been critical of the Islamic Republic.