A recently assassinated Iranian nuclear scientist was killed by killed by “agents of imperialism and international Zionism,” Mahmoud Ahmadinejad wrote Friday in a letter to the family of Mostafa Ahmadi, the target of a bombing attack earlier this week.

Want to know more about Iran's nuclear standoff with the West? Join Haaretz.com's official Facebook page

The letter, which was reported by the official Iranian news agency IRNA, came as the country’s state television quoted Iranian Supreme Leader Ayatollah Ali Khamenei as blaming Israel and the U.S. for the Tehran attack.

“This cowardly [act of] terror, whose perpetrators and plotters will never dare to confess to their dirty and hideous crime or accept responsibility for it, has been carried out by the planning or support of CIA and Mossad services, like all other crimes of the network of international state terrorism,” Khamenei was quoted as saying by Press TV.

Khamenei added that the assimilation indicated that “the global arrogance spearheaded by the US and Zionism has reached a deadlock in confrontation with the determined, devout, and progressive nation of Islamic Iran,” adding that Iran's scientific progress was not dependent on individuals but was a “historic movement” rising from the “invulnerable national resolve” of Iranians.

The Iranian Supreme Leaders spoke after a hard-line Iranian newspaper called Thursday for retaliation against Israel, a day after the mysterious killing of a nuclear scientist in Tehran with a magnetic bomb attached to his car.

A column in the Kayhan newspaper by chief editor Hossein Shariatmadari asked why Iran did not retaliate.

"Assassinations of Israeli military and officials are easily possible," he wrote. The attack - which instantly killed the scientist and his driver on Wednesday - was at least the fourth targeted hit against a member of Iran's nuclear brain trust in two years.

Secretary of State Hillary Rodham Clinton denied any U.S. role in the slaying, and the Obama administration condemned the attack.