Mohammadi, a lecturer at Tehran university, died when a bomb strapped to a motorcycle was triggered by remote control outside his home in the northern Tehran neighbourhood of Qeytariyeh, state media said.
Iran's state broadcaster said Mohammadi was "assassinated in a terrorist act by counter-revolutionaries and agents of the arrogance," without naming any sources.
Iranian officials usually refer to the United States and some other Western powers as the "global arrogance."
Tehran prosecutor Abbas Jafari Dolatabadi told the ISNA news agency that Mohammadi was a lecturer in nuclear energy and said the booby-trapped bike was parked outside his house and exploded as he was getting into his car.
"The judiciary has launched an investigation ... no suspects have yet been arrested" he said.
Bomb attacks are rare in Iran although several security officials and members of the elite Revolutionary Guards have been killed in bombings by rebels in restive Sistan-Baluchestan province in eastern Iran.
A witness told AFP that Tuesday's explosion was a "strong blast breaking window glasses in neighbouring houses and cars."
Iran's state-run Arabic-language TV Al-Alam identified Mohammadi as a "hezbollahi" teacher -- a term used for staunch supporters of the Iranian regime.
"This assassination may have been carried out by the Hypocrites (Iran's exiled People's Mujahedeen opposition) or planned by the Zionist regime," Al-Alam said.
Iranian authorities have consistently accused archfoes the United States and Israel of seeking to foment unrest in Iran.
Hardliners have also accused the People's Mujahedeen of infiltrating anti-government protests and carrying out attacks.
None of the reports said whether Mohammadi was connected to Iran's controversial nuclear programme, which the West suspects is masking an atomic weapons programme.
Iran has been under international pressure to halt its sensitive uranium enrichment programme which is at the centre of fears about Iran's ambitions as the process which makes nuclear fuel can also be used to make atom bombs.
Despite three rounds of UN Security Council sanctions, Iran has continued to expand its nuclear programme.
World powers seeking to curb Iran's atomic drive are still awaiting a response from Tehran to a UN-brokered offer to ship most of Iran's low enriched uranium abroad to be further refined into reactor fuel.
Iran has rejected the offer and has offered its own counter-proposal of a staged swap.
Some Western powers have dismissed the Iranian proposal and insist the Islamic republic accept the UN-backed deal or face further sanctions.
Source: AFP Global Edition