Academics in Iran have been left in a state of critical fear following the murder in Tehran yesterday of nuclear physicist Majid Shahriari and the attempted assassination of another nuclear researcher, Fereydoon Abbasi.

The separate attacks occurred yesterday morning and both were carried out by unidentified assailants on motorbikes who attached explosives to the victims' vehicles as they travelled through the capital. Both scientists were based in the faculty of nuclear engineering at the Shahid Beheshti University in Tehran and they are both said to be key figures in Iran's controversial nuclear programme.

Iranian president, Mahmoud Ahmadinejad, immediately blamed the attacks on foreign enemies saying that "undoubtedly the hand of the Zionist regime and Western governments is involved". As yet, however, no nation or group has claimed responsibility.

"Everyone is shocked. Programmed assassination of scientists is the last thing we could imagine," says Reza Mansouri, a cosmologist at Sharif University in Tehran and a former Iranian deputy science minister. He confirmed that the widely held view in Iran is that this attack was carried out by foreign agents.

Medical physicist?

Shahriari was a nuclear researcher and his publication record suggests that he specialized in medical applications of nuclear physics, with a particular interest in modelling neutron transport processes. His role in Iran's nuclear programme was confirmed by Akbar Salehi, head of Iran's atomic energy agency, who says that Shahriari was involved in "one of the great projects" of the agency. "Majid Shahriari was one of my students for years and had a good cooperation with the organization," Salehi told the Islamic Republic News Agency.

Fereydoon Abbasi, the scientist who survived, appears to have a larger involvement with Iran's nuclear programme. In 2007 his name was added to the United Nations Security Council's sanctions list for his involvement in "nuclear or ballistic missile activities".

Monday's attacks follow the assassination in January of another Iranian physicist, Masoud Alimohammadi, a quantum mechanics and field theory specialist at Tehran University, who was killed by a remote-controlled bomb attached to the side of a motorcycle. The similarities between the attacks have left Iranian academics in an acute state of fear.

Looking for a motive

Despite the government's conviction that the murders have been carried out by Israeli or US special services, there is also speculation that the attackers could have belonged to one of a number of organizations within the Middle East. The speculation is fuelled by the continuing secrecy that surrounds Iran's nuclear programme and the increasingly complicated political situation in the Middle East.

Similarly, the motive for assassinating these particular men is not clear, particularly given their seemingly disparate research interests. One thing they do share is that all three men held roles in SESAME – Synchrotron-light for Experimental Science and Applications in the Middle East – a project that aims to create the region's first major international research centre by building a synchrotron light source in Jordon. Shahriari is listed as having been an adviser to the project, while Abbasi is a member of the Iranian advisory committee to SESAME, and Alimohammadi was a member of the council.

SESAME president Chris Llewellyn Smith, who is also a former director-general of CERN, says that he does not remember Shahriari though the official records state that he did attend one council meeting. Llewellyn Smith does, however, recall meeting Alimohammadi but he, likewise, was only able to attend one meeting before he was killed.

Llewellyn Smith is keen to point out that synchrotrons such as SESAME are not in any sense nuclear facilities. "They are accelerators that are designed to produce intense light with wavelengths ranging from the infrared to X-rays, which are used to study matter on scales ranging from biological cells to atoms," he says.