TEHRAN, Iran — Iran said Saturday it has evidence that the United States was behind the assassination of an Iranian nuclear scientist this week in Tehran, state media reported.

Mostafa Ahmadi Roshan was killed in a daylight assassination 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 program, 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.