Mostafa Ahmadi Roshan, a deputy director of Iran’s largest uranium enrichment plant, died in a car bomb explosion in Tehran on Wednesday. Local police sources said a motorcyclist attached a magnetic mine to his car.

"This cowardly murder, perpetrated by those who do not dare claim their abominable crime by accepting the responsibility, was committed like the other crimes with the planning or support of the intelligence services of the CIA and Mossad," Khamenei said in a message of condolences to Roshan’s family on Thursday.

“Such criminal acts show that the Imperialism and global arrogance headed by the US and international Zionism have come to a dead-end in their confrontation with the determined and pioneer Iranian nation,” he said.

The United States has already denied having anything to do with the Iranian scientist's murder, while Israel has not commented on its alleged involvement in the assassination.

The attack on Wednesday was similar to several other attacks on Iranian nuclear scientists in the past two years.

In January 2010 Professor Mahsud Ali Mohamed of Tehran University was killed outside his home. Local police say he died when a motorcycle exploded next to his car as he got into the vehicle.

In November of that year, bomb attacks killed two atomic scientists, Majid Shakhriani, and his colleague Dr. Fereidoun Davani-Abbasi, and injured their wives. In both cases, their cars were attacked by unknown motorcyclists.

Western nations claim Iran’s nuclear energy research is a cover for atomic weapons production, which Tehran has consistently denied.

Iran has recently begun enrichment of uranium to 20 percent at its Fordo facility as part of its nuclear program, the IAEA said on Monday. Russia said it was concerned by the move, which was condemned by the Western powers.