Senior Iranian officials have blamed the remote-controlled blast on an exile opposition group they claim has ties to the US and Israel.
Mohammad Khatami and Hashemi Rafsanjani, both former Iranian presidents, condemned the attack in separate statements on Wednesday.
They did not directly accuse any group of the killing, but each honoured Ali Mohammadi - a 50-year-old researcher and lecturer in quantum physics - as a victim of terrorism.
Khatami, a strong backer of Mir Hossein Mousavi, has denounced the escalating crackdowns on opposition protesters and said the bomb attack was carried out by groups seeking to "further destabilise the crisis".
In a statement of condolence to Ali Mohammadi's wife posted on Khatami's website, he said the "ugly face of terrorism" was behind the bombing.
"The dirty hand who did this murder with no doubt was the enemy of Iran," Khatami wrote.
Rafsanjani's statement, carried by the semi-official ILNA news agency, called the bombing "cowardly terrorism" and a sign of "a new era of intrigue" in Iran.
The statements by Khatami and Rafsanjani added to the puzzling array of claims and clues following the attack.
The US state department has dismissed accusations that the US would be involved in the attack. Mark Toner, a US state department spokesman, called the allegations "absurd".
But the media adviser to Ahmed Ahmadinejad, the president, repeated the accusations against the US and Israel on Thursday and said Britain was also "high on
the list of suspects."
"Our security and intelligence apparatus are pursuing those behind the blast to bring them to justice as soon as possible," Ali Akbar Javanfekr told the AFP news agency.
"America, Britain and Israel have the most animosity towards our scientific progress, especially the nuclear programme, so when our scientists are targeted they become high on the list of suspects," he said.
The pro-government militia group, the Basij, has also praised Ali Mohammadi as a martyr.
No obvious motive
Ali Mohammadi was not known to have any important roles in the opposition movement, although his name appeared on a university petition pledging support for Mousavi, the reformist candidate, before last June's disputed election.
Some reports say he worked for the Revolutionary Guards for more than 20 years until 2003.
But Al Jazeera's Alireza Ronaghi, reporting from Tehran, quoted one of Ali Mohammadi's close friends as saying that the professor was never a political activist.
"He told me that Ali Mohammadi had very deep reformist tendencies but never mixed it up with his professional character."
Ali Moghara, who heads the physics faculty at Tehran University, said Ali Mohammadi was just a "world famous" physicist who engaged in "no political activity".