Mostafa Ahmadi Roshan was killed in a brazen daylight assassination on Wednesday when two assailants on a motorcycle attached a magnetic bomb to his car in the Iranian capital.
The killing bore a strong resemblance to earlier killings of scientists working on the Iranian nuclear programme, and has prompted calls in Iran for retaliation against those deemed responsible.
The IRNA state news agency said Saturday that Iran's Foreign Ministry has sent a diplomatic letter to the U.S. saying that it has "evidence and reliable information" that the CIA provided "guidance, support and planning" to assassins "directly involved" in Roshan's killing.
The U.S. has denied any role in the assassination. Iran delivered the letter to the Swiss Embassy in Tehran, which looks after U.S. interests in the country. Iran and the U.S. have had no diplomatic relations since the 1979 Islamic Revolution.
IRNA also reported that Iran delivered a letter to Britain accusing London of having an "obvious role" in the killing.
It said that a series of assassinations began after British intelligence chief John Sawers hinted in 2010 at intelligence operations against the Islamic Republic.
British media have quoted Sawers as saying that intelligence-led operations were needed to make it more difficult for countries like Iran to develop nuclear weapons. Israeli officials, in contrast, have hinted at covert campaigns against Iran without directly admitting involvement.
The killing has sparked outrage in Iran, and state TV broadcast footage Saturday of hundreds of students marching in Tehran to condemn Roshan's death and calling for the continuation of the country's disputed nuclear programme.