Tehran, Iran (CNN) -- A group "active in the post-election riots" in Iran claimed it assassinated a nuclear physics scientist on Tuesday, an Iranian media outlet said.

Fars News Agency said the Royal Association of Iran accepted responsibility for the killing of Massoud Mohammadi in a bomb blast near his home in northern Tehran.

"Members of this association had infiltrated into the country and had been active in the post-election riots of last June and played a part in setting public property on fire and inciting people to riot," Fars reported.

"It had been established during trials (of some of the members of this association) it had been established that they were in charge of showing people how to make Molotov cocktails and setting public places on fire."

An Iranian Foreign Ministry spokesman also cast suspicion on Israel, the United States and their allies. And Press TV correspondent Amir Mehdi Kazemi, reporting from the scene of the assassination, quoted security sources "as saying that the equipment and system of the bomb used in the attack had been related to a number of foreign intelligence agencies, particularly Israel's Mossad."

Tehran's Prosecutor Abbas Jafari-Dolatabadi said Mohammadi taught neutron nuclear physics at Tehran University, Press TV reported.

Head of the Science Complex of the University of Tehran Ali Moqari told Mehr News Agency on Tuesday that Mohammadi "was a prominent international figure, but had not been involved in any political activities." He said the professor was prominent in his field and had written books.

However, State-run Press TV reported that Mohammadi was a staunch supporter of the Islamic Revolution, which saw Ayatollah Khomeini seize power from the ruling monarchy in 1979 and declare Iran an Islamic Republic with himself as supreme leader.

News agencies reported conflicting accounts of Mohammadi's death. Fars said the bomb was placed in a garbage can outside the professor's house. Press TV said the bomb was placed inside a motorbike parked outside his house in the Qeytariyeh neighborhood in northern Tehran and was remotely detonated.

According to Fars, the Royal Association's statement said Mohammadi played "an active role during the recent disturbances in Tehran, dresses as a plainclothesman," a common reference to police and others working with security forces to quell riots.

"We declare with pride that we are Mohareb (those who wage war against God), yes, we are Mohareb," the association said, according to Fars.

Grass-roots outrage has swept Iran over the disputed presidential elections in June, when incumbent Mahmoud Ahmadinejad won a second term. Hundreds of thousands of Iranians took to the streets to demonstrate against the official result, and that prompted a forceful and sometimes deadly government crackdown.