“We have reliable documents and evidence that this terrorist act was planned, guided and supported by the CIA,” the Iranian foreign ministry said in a letter handed to the Swiss ambassador in Tehran, state TV reported.

“The documents clearly show that this terrorist act was carried out with the direct involvement of CIA-linked agents.”

The Swiss Embassy has represented U.S. interests in Iran since Iran and the U.S. cut diplomatic ties shortly after the 1979 Islamic revolution.

State TV said a “letter of condemnation” had also been sent to the British government, saying the killing of Iranian nuclear scientists had “started exactly after the British official John Sawers declared the beginning of intelligence operations against Iran”.