Sixty-two people were involved in the plot, according to Mohamad Ahwaze, an Iranian journalist who reported the extent of the COVID-19 pandemic in his country.
Ahwaze said he had received secret Iranian information.
Iranian leaders have accused Israel's Mossad of the killing. One American official and two other intelligence officials also told the New York Times that Israel was in charge of the attack.
Ahwaze said that 12 members of the team, who he portrayed as being highly prepared and assisted by "security and intelligence services abroad', were deployed to the city of Absurd, 50 miles east of Tehran.
The hill retreat of 10,000 people is where many Tehranis have second homes, and Fakhrizadeh, 59, had a home there.
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Another 50 people, Ahwaze said, assisted with logistical support. He did not specify whether they were in Iran, or elsewhere.
The team had been watching Fakhrizadeh and knew that he was going to be driving from Tehran to Absard on Friday.
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They plotted the attack for a roundabout in Absard, at the foot of a tree-lined street that enters the city. four motorcycles and two snipers were expecting him at the scene of the trap, along with a booby-trapped Nissan truck.
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Half an hour before Fakhrizadeh's convoy of three bulletproof cars came, the power was cut off to the area, Ahwaze reported.
The team was in place when the first car reached the roundabout.
As the third car passes, the Nissan explodes, damaging electricity poles and transmitters, according to a state TV report from the area on Friday night.
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The energy of the explosion from the bomb caused wreckage at least 300 meters, according to state television.
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The second car, carrying Fakhrizadeh, was then fired at by the 12 assassins, including two snipers.
The gunmen with the assassination team opened fire on the cars, and an intense gunfight ensued, according to Sepah Cybery, a social media channel linked to the Islamic Revolutionary Guards Corps.
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Ahwaze tweeted: "According to Iranian leaks, the leader of the assassination team took Fakhrizadeh out of his car and shot him and made sure he was killed."
The assassination team then disappeared, having sustained no losses to their team, Ahwaze reported.
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A spokesperson for the Atomic Energy Organization in Iran stressed that no accident had occurred and all scientists were safe and well.
Israeli Prime Minister Benjamin Netanyahu named Fakhrizadeh-Mahabadi as boss of the SPND during a news conference in 2018.
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