Quoting an anonymous Israeli intelligence source, Germany's Spiegel Online said the Mossad -- Israel's secret service -- was behind the July 23 assassination of Iranian nuclear scientist Darioush Rezaei.
Two unknown assailants shot Rezaei, 35, in front of his daughter's kindergarten in east Tehran, afterward escaping on a motorcycle.
Rezaei had been developing high-voltage switching systems used to detonate a nuclear warhead.
"That was the first serious action taken by the new Mossad chief, Tamir Pardo," the Internet edition of DerSpiegel quoted the Israeli source.
"As long as [the] Mossad is leading the fight against the bomb, it will get the big budgets," the website quoted the Israeli source.
"Whether there will be an open attack on Iran's nuclear facilities in the future will partly depend on whether the Israeli military or intelligence wins the internal power struggle," he said.
Rezaei was the third Iranian nuclear physicist to be assassinated since the beginning of 2010.
In January 2010, senior nuclear physicist Masoud Ali Mohammadi was killed when an explosives-laden motorcycle blew up alongside his car.
On November 29, 2010, nuclear physics professor Majid Shahriari died when his car exploded. Shahriari specialized in neutron transport - a major component in the manufacture of nuclear bombs.
Fereidoun Abbassi -- an expert in the separation of nuclear isotopes -- survived an attempt on his life. Afterward, Iranian President Mahmoud Ahmadinejad appointed him head Iran's Atomic Energy Organization.
Iranian parliamentarian Kazem Jalali said the assassinations proved the U.S. and Israel are "desperate" to stop Iran's nuclear program.
A State Department spokeswoman said the U.S. was not involved in Rezaei's assassination, DerSpiegel reported.
Last summer, Israel was blamed for the Stuxnet computer worm that caused havoc at several Iranian nuclear plants.