On the choking, gridlocked streets of Tehran, motorcycles are taking on an increasingly sinister aura. It is not just military officials and government scientists who look twice in their wing mirrors when they see a motorbike sidle up alongside them. What was once considered plain practical or occasionally hip has now been tainted by its new role as a messenger of death.

When Mostafa Ahmadi-Roshan, a 32-year-old chemist, was killed in Wednesday-morning traffic by motorcycle-borne assassins, it was the fifth time in two years that a scientist from the state nuclear programme had been targeted. Each time, the hit squad used a motorcycle.

In a crowded city such as Tehran, it is the ideal assassin's tool. Its rider can be masked without appearing out of the ordinary; it can edge closer to its target without attracting attention, and it can also ensure a quick getaway even in the middle of rush hour, flying down the channels between the cars and instantly blending in with a million other similar vehicles.

For some of the same reasons, the regime had used motorcycles against its own people, unleashing bike-mounted black-shirted thugs, wielding blades to cut through crowds of opposition demonstrators. There are some inside and outside Iran who speculate that the assassinations could be carried out by the Iranian "deep state" aimed at nuclear scientists it suspects of disloyalty.

The first victim of the scientist murders, Masoud Ali-Mohammadi – who was killed in January 2010 by a bomb on a parked motorcycle as he walked from his house to his car – was indeed a follower of the opposition Green movement, as well as a senior official in the Institute of Applied Physics (IAP), where western officials suspect a lot of research work is done. So, both sides arguably had a motive for killing him.

However, there has been no mistaking the fury of the Tehran government in the two years since as, one by one, its scientists – the crown jewels of its nuclear aspirations – have been picked off. It would be a huge irony if the killers were simply emulating the Iranian security apparatus.

The next attacks came in November 2010. In simultaneous and identical attacks, two motorcycles pulled alongside two cars in different parts of Tehran, the pillion passenger clamped a magnetic bomb to the door next to their intended victim and sped away. One target, Majid Shahriari, another senior scientist in the nuclear programme, was killed instantly. The other, Fereydoun Abbasi-Davani, survived – the only survivor of the five attacks to date. He was also a big fish at the IAP, and was made head of the overall Iranian atomic programme soon after he recovered from the blast.

Target number four was Dariush Rezaeinejad, an electronics expert working for the nuclear programme, reportedly working on high-voltage switches, a dual-use item that can be used in nuclear warheads. He was shot in Tehran by two unknown gunmen, who again arrived and left on motorbikes.

Wednesday's killing of Ahmadi-Roshan was a carbon copy of the November attacks on Shahriari and Abbasi-Davani: the masked helmeted assassins on their bikes, a limpet bomb shaped to do maximum damage inside the car, minimum damage outside. Even the victim's car was the same, a silver Peugeot 405.

The Iranian government has blamed the US and Israel for the murders, mostly Israel, and the Israelis themselves are doing little to deflect that blame. While the White House and US state department denounced the killing, the Israeli military spokesman, Brigadier General Yoav Mordechai, remarked on Facebook: "I don't know who took revenge on the Iranian scientist, but I am definitely not shedding a tear."

Israel certainly has a track record of assassinations, from the Palestinian perpetrators of the Munich Olympic attack of 1972, to the killing of senior Hamas official Mahmoud al-Mabhouh in a Dubai hotel room in early 2010. This week, Le Figaro reported that Mossad had been recruiting Iranian dissidents in the Kurdish region of neighbouring Iraq, to conduct operations in Iran.

"If you look at the choice of target it really could only be Israel," says Robert Baer, a former CIA agent in the Middle East, currently working on a book on assassination called The Perfect Kill. "If it was an internal group, like the MeK ([Mujahedin-e-Khalq] it would be security official or policeman who had been torturing their guys. If you look at the motivation, it must be Israel."

However, Baer adds that it is quite likely that Israel is acting in tandem with an Iranian dissident organisation. "To do this in the middle of the day, with a limpet charge and then getaway, you need a lot of people on the ground," he says. " You need an extensive network of the kind only someone like MeK can provide."

An executive order dating back to the presidencies of Gerald Ford, Jimmy Carter and Ronald Reagan prevents the CIA from carrying out political assassinations. Exceptions have been made for Taliban and al-Qaida suspects on the grounds that the US is at war with those organisations.

"We are not in a war with the Iranians, so if we were involved in any way we would be culpable," says William Banks, an expert on national security law at Syracuse University. "The executive order can be secretly waived by the president, but then there are questions of international law and problems with our European allies. Lashing out at those we are not at war with would also be a moral issue, and would invite reciprocity."

All the targets of the assassination campaign appear to have been carefully chosen. They were suspected of involvement in nuclear weaponisation, or in Ahmadi-Roshan's case, the procurement of equipment for the Iranian uranium enrichment project, which has been outlawed by the UN security council.

However, few observers of the Iranian nuclear programme believe that the killings will seriously impede Iran's progress, either towards a nuclear weapon, as the west maintains, or towards a civilian nuclear industry, as Tehran insists.

David Albright, the head of the Institute for Science and International Security, says: "The effect is not so much eliminating people, as they are replaceable. It is a classic terror tactic, designed to make people feel their own government can't protect them. It prevents people from participating and so leads to a loss of recruitment."

Baer argues that the impact on the nuclear programme itself is likely to be so minimal, it is unlikely to be the aim of the murder campaign.

"It's a provocation," he says. "My theory is that Israel couldn't get the White House to agree to bombing. It is not satisfied with sanctions, so the Israelis are trying to provoke the Iranians into launching a missile and starting a war."

Over two years, five attacks and four killings, Tehran has held its nerve, but there were calls in the conservative Tehran press for revenge yesterday. And in the Washington bomb plot last October and the storming of the UK embassy in November, there are increasing signs that individual elements inside the regime, in the Revolutionary Guards for example, are making their own decisions.