Iran believes Israel and an exiled opposition group used a remote-control weapon to shoot dead top nuclear scientist Mohsen Fakhrizadeh on Friday.
Security chief Ali Shamkhani said the attackers had “used electronic equipment” when Fakhrizadeh’s car was fired on east of the capital Tehran.
He was speaking at the funeral of the scientist Israel accused of secretly helping to develop nuclear weapons.
Israel has not publicly commented on the allegations of its involvement.
In the early 2000s, Fakhrizadeh played a crucial role in Iran’s nuclear programme but the government insists its nuclear activities are entirely peaceful.
It has been subjected to crippling Western sanctions aimed at preventing it from developing nuclear weapons.
How did the scientist die?
Iranian versions of what happened have changed significantly but it appears that Fakhrizadeh was mortally wounded when his car was sprayed with bullets in the town of Absard, to the east of Tehran.
During the attack a bomb in a Nissan pickup truck is also reported to have exploded.
Pictures on social media show a road strewn with wreckage and blood, and a bullet-riddled vehicle.
First the defence ministry reported a gunfight between Fakhrizadeh’s bodyguards and several gunmen.
One Iranian report quoted witnesses as saying “three to four individuals, who are said to have been terrorists, were killed“.
Then Iranian media said the scientist had in fact been killed by a “remote-controlled machine gun” or weapons “controlled by satellite”.
And on Monday, Rear Admiral Shamkhani, who heads the Supreme National Security Council, confirmed it had been a remote attack, using “special methods”.
“It was a very complex mission using electronic equipment,” he said at the funeral. “There was no-one present at the scene.”
He said Iranian intelligence and security services had been aware of a plot to assassinate Fakhrizadeh, and had even predicted where the attack might take place.
On who was to blame, he singled out exiled Iranian opposition group the Mujahideen-e Khalq and Israel.
Israel’s Intelligence Minister, Eli Cohen, said on Monday in an interview with a radio station that he did not know who was behind the killing.
However, an unnamed senior Israeli official involved in tracking Iranian nuclear activities was quoted by the New York Times as saying that “Iran’s aspirations for nuclear weapons, promoted by Mr Fakhrizadeh, posed such a menace that the world should thank Israel”.
Machine guns and other remotely controlled ground weapons are now widely used across the Middle East, according to a report by Forbes.
They are employed both by professional armies, such as those mounted on combat vehicles, but also by militants who are known to have put them in vehicles or stationary posts.