The official explanation for the Friday killing of Fakhrizadeh continued to change on Monday, with the secretary of Iran's Supreme National Security Council, Ali Shamkhani, saying the operation "was very complex, using electronic equipment and no one was present at the scene".
Iran's semi-official Fars news agency said on Sunday Fakhrizadeh was assassinated by an automatic machine gun operated with a remote control.
Iran on Monday held a funeral service for the scientist who founded the country's military nuclear program, and repeated previous claims that the assassination had been carried out by Israel.
State TV's English-language broadcaster Press TV reported earlier Monday that a weapon recovered from the scene of the attack bore "the logo and specifications of the Israeli military industry". It cited an unidentified person and didn't provide any evidence to back up the claim.
Hajizadeh also said that the martyrdom of Fakhrizadeh would strengthen the resolve of the late scientist's colleagues and friends to press on. "Iran leaders would be wise to wait for the return of responsible American leadership on the global stage & to resist the urge to respond against perceived culprits".
Iran gave the status of "martyr" to Fakhrizadeh, while stating it would double its efforts to continue its nuclear programme.
Eyewitnesses testified to the presence of assassins to state television on Friday, while Fars News initially reported that "three or four terrorists" were killed in a firefight.
The nuclear watchdog has provided an unprecedented, real-time look at Iran's civilian nuclear program following the country's 2015 nuclear deal with world powers.
The man Israel alleges was the father of an Iranian nuclear weapons programme was senior enough to meet with supreme leader Ayatollah Ali Khamenei in January 2019, based on official pictures released after his death.
Tehran has always denied seeking nuclear weapons.
In March 2007, Fakhrizadeh was targeted by United Nations Security Council sanctions along with other "persons involved in nuclear or ballistic missile activities" for Iran.
"The enemies know and I, as a soldier, tell them that no crime, no terror and no stupid act will go unanswered by the Iranian people", said Defence Minister Brigadier General Amir Hatami in a televised speech at the ceremony.
Mr Hatami also criticised countries that had not condemned Mr Fakhrizadeh's killing, warning: "This will catch up with you someday".
According to a former head of Israel's military intelligence, the senior Iranian scientist assassinated last week was such a central figure in Tehran's nuclear program that it would be almost impossible to replace him and thus is liable to significantly set back the Islamic Republic's atomic ambitions.
Overnight, the United Arab Emirates, which just reached a normalisation deal with Israel, issued a statement condemning "the heinous assassination". The UAE, home to Abu Dhabi and Dubai, warned the killing "could further fuel conflict in the region".
Mr Hatami also called the nuclear arsenal of the U.S. - and the stockpile of atomic bombs Israel long has been suspected of holding - "the most unsafe threat against humanity".
