Iran's final investigation into the May helicopter crash that killed President Ebrahim Raisi has found it was caused by bad weather, the body investigating the case said Sunday.

The helicopter carrying 63-year-old Raisi and his entourage came down on a fog-shrouded mountainside in northern Iran, killing the president and seven others, and triggering snap elections.

The main cause of the helicopter crash was the "complex climatic and atmospheric conditions of the region in the spring", the special board investigating the dimensions and causes of the helicopter accident said, according to state broadcaster IRIB.

The report added that "the sudden emergence of a thick mass of dense and rising fog" caused the helicopter's collision into the mountain.

Iran's army in May similarly said it had found no evidence of criminal activity in the crash that also killed Raisi's foreign minister, Hossein Amir-Abdollahian.

In August, Fars news agency cited the main causes of the May 19 crash as bad weather conditions and the helicopter's inability to ascend with two extra passengers against security protocols.

But the Iranian armed forces were quick to reject the finding saying, "what is mentioned on Fars news about the presence of two people in the helicopter against the security protocols... is completely false".