LONDON (Reuters) - A backseat passenger on a motorcycle weaving through the crush of Tehran's morning traffic reaches out and places a small magnetic device on the door of a silver-grey Peugeot 405.
When the directional bomb explodes seconds later, blasting through the sedan's door and instantly killing nuclear scientist Mostafa Ahmadi-Roshan, a 32-year-old father of one, the motorcycle has already vanished, accelerating into the ranks of the Iranian capital's rush hour.
The proficiency of the latest assassination to deplete Iran's community of atom specialists suggests that violent actions by one or more of Iran's adversaries form an increasingly active - and public - element in a multifaceted international drive to impede Iran's nuclear program.
Some old espionage hands voice respect for the expert landing of clandestine, deniable blows against a program the West suspects is aimed at acquiring a nuclear bomb capability and Iran says is for civilian purposes.
"Ten out of 10. They hit the target and nobody got caught," former U.S. intelligence officer Robert Ayers told Reuters of the January 11 killing. "What makes these things so impressive is they gather a lot of information and do their 'on the ground' homework, which can take months."
Sidney Alford, a British explosives expert, says the hit was technically "professional. It worked and it worked very well."
But whoever the apparently adept perpetrators were, the attack appears to form part of a quickening series of sabotage and assassinations that is growing less covert by the month.
And the more visible the cloak-and-dagger campaign grows, some analysts argue, the more acute its affront to national prestige and sovereignty, and the deeper the siege mentality widely held to motivate Iran's drive for nuclear prowess.
Ahmadi-Roshan was the fourth Iranian nuclear scientist killed in the past two years; another scientist survived an explosion that wounded him and his wife.
Iran says scientists have also been kidnapped, a computer virus attacked its nuclear equipment, and a massive explosion at a military base, which Iran called an accident, killed more than a dozen officers including the head of the Revolutionary Guards missile program.
The campaign, coinciding with a toughening of economic sanctions, may strain any discreet diplomatic feelers between Tehran and Washington, some Western analysts say.
Iran is in defiant mood.
"If Israel thinks they can prevent our studies with four terrorist attacks, it's a very weak way of thinking... Everybody will learn that they can't stop us with such actions," said Iran parliament speaker Ali Larijani the day after the killing.
Ali Vaez and Charles D. Ferguson of the Federation of American Scientists wrote that "such acts of terrorism" are unlikely to significantly delay or deter Tehran's nuclear work.
"The resulting climate of insecurity feeds ammunition to hardliners in Tehran demanding reprisals."
Ahmadi-Roshan's killing happened less than two weeks after the Obama administration signed into law an unprecedented tightening of sanctions aimed at Iranian oil exports.
To some, the evident effectiveness of tougher sanctions in getting the attention of Tehran's leaders might obviate at least for the moment any need for a resort to clandestine methods.
In response to a new U.S. law targeting Iranian oil income Tehran threatened to choke the West's supply of Gulf oil if its exports are hit. Washington warned that the U.S. navy was ready to open fire to prevent any blockade of the Strait of Hormuz, through which a third of the world's seaborne traded oil passes.
"In this process of ever-accelerating sanctions, we have arrived at a point where sanctions begin to blur into actual warfare," wrote Iran expert and former U.S. official Gary Sick.
"If the sanctions succeed in their purpose of cutting off nearly all oil exports from Iran, that is the equivalent of a blockade of Iran's oil ports, an act of war."
Meanwhile, spectacular mishaps in Iran's nuclear program or military facilities appear to be multiplying, in tandem with a series of espionage-related incidents that have raised the diplomatic temperature, including an Iranian court's sentencing of an Iranian-American man to death for spying and the apparent malfunctioning and crash in eastern Iran of a U.S. drone.
The attacks are making some in the West uncomfortable.
Hans Blix, director general of the International Atomic Energy Agency in 1981-97 and a former Swedish foreign minister, told Reuters: ""When it comes to the murder... What is the effectiveness of it?
"I think people will be indignant, and in fact not only in Iran. I think people everywhere are indignant."
The result of more frequent and public attacks could be increased tension, analysts say, raising risks of a clash between U.S. and Iranian forces in the Gulf or of a unilateral Israeli air strike on Iran's nuclear sites, either one of which might result in temporary closure of the strategic waterway.
Iranian officials remember well that before Israel's 1981 air strike on a nuclear reactor in Iraq, there were similar acts of sabotage and assassination attributed largely to Israel.
John Cochrane, a defense specialist at London-based Exclusive Analysis, told Reuters that the killings in Iran could be seen as effective "in the narrow sense" that they sought an erosion in Iran's nuclear expertise.
"But clearly the big risk is that the Iranians are quick to point the finger at Israel or the U.S., so there is no particular restraint on their (Iranian) side from carrying out some particular asymmetric attack which has the risk of producing a spiral of violence."
"Israel is the key player. It is the state that sees itself as under existential threat and has the capacity, just, to exercise a strike option."
Metsa Rahimi of Janusian security consultants in London said the killings had failed to deter Iran's nuclear program since "the Iranian regime's will is made of stronger stuff and most (of its leaders) would probably say that the death of a few scientists will not be decisive in this game."
The day after the assassination, Iran's parliament speaker Ali Larijani reiterated on a visit to Turkey that Iran wanted to restart negotiations with six world powers to resolve the nuclear row. The last talks collapsed a year ago.
Western countries have so far refused Iran's proposal for more talks, arguing that it is a waste of time because Tehran will not discuss halting its uranium enrichment.
Speculation has lingered about a possible divergence of views between the United States and Israel on tactics. U.S. Secretary of State Hillary Clinton responded to the January 11 killing by saying the United States had nothing to do with any "violent acts inside Iran" and condemning such actions.
U.S. Iran expert Sick wrote: "The U.S. government had made no such intervention in previous assassination cases. If the perpetrator was, as widely suspected, Israel, this was a serious warning not to interfere in U.S. diplomatic efforts."
He wrote that while the Israeli government distrusted the diplomatic track, the Obama administration had looked hard at the potential effects of a war with Iran and "has decided that a return to the negotiating track is essential."
Asked where the Iran standoff was heading, Blix replied: "For the moment the decibel level is fairly high. But it is clear to me that the Obama administration, that cannot allow itself to be described as soft and that says that 'all options are on the table'... that the Obama administration does not want war and bombing. That is quite clear."
"The American public is clearly somewhat war-fatigued."
Israel and its main allies are on common ground on much when it comes to Iran. Israel, the United States and Britain have all made clear that they view covert operations as a sensible alternative to conventional military action.
Last year's Stuxnet computer worm, which damaged computers used in industrial machinery, was widely believed to have been a U.S.-Israeli attack to cripple Iranian nuclear centrifuges.
In a speech at Reuters London offices in 2010, John Sawers, overseas espionage chief of U.S. ally Britain, made an unusually forthright comment on the topic, saying that stopping nuclear proliferation could not be done just by conventional diplomacy.
"We need intelligence-led operations to make it more difficult for countries like Iran to develop nuclear weapons. The longer international efforts delay Iran's acquisition of nuclear weapons technology, the more time we create for a political solution to be found."
But it is not clear that the United States and its European allies believe subversion acts involving violence are prudent.
One former senior European intelligence strategist told Reuters killings were "an unacceptable tactic."
The scars of the Iraq war, which was launched on information about Iraq's weapons of mass destruction programs that turned out to be false, run very deep.
A 2011 RAND Corporation study led by former U.S. diplomat James Dobbins said that U.S. military options apart from conventional air strikes included "show-of-force operations in the Persian Gulf, cyberwarfare, and a broad-based air campaign against political and military targets."
But Dobbins's report argues that while covert action might slow the Iranian nuclear program it is unlikely to stop it and might have "the unintended consequence of fortifying the regime's resolve in continuing the nuclear program."
Israel does not comment directly on covert operations but it is suspected by some of viewing more favorably than its allies covert actions that risk or seek to inflict bloodshed.
Israel says it has no option but to take seriously appeals by Iranian leaders for Israel's demise, calls that have prompted Prime Minister Benjamin Netanyahu to liken them to the Nazis.
And to those who object to assassination on moral grounds, Israel's supporters such as Louis Ren Beres, Professor of International Law at Purdue University, Indiana, say such targeted killings may be justified in self-defense.
"As long as Iran proudly announces its literally genocidal intentions toward Israel, while simultaneously and illegally developing nuclear weapons and infrastructures, Jerusalem has no reasonable choice but to protect itself with the best means available," he wrote.
Any Israeli pre-emptive measures, he wrote, would perhaps involve "the targeted killing of selected enemy scientists or military figures and substantially expanded cyber-warfare."
Israel's intelligence minister Dan Meridor distanced himself from the January 11 killing, saying "I don't know this subject."
But at other times Israeli officials have sometimes reacted to news of the periodic mishaps in Iran's nuclear program by issuing denials or comments that have bordered on the laconic.
"I don't know who settled the score with the Iranian scientist, but I am definitely not shedding any tears," Israel's military spokesman Brigadier-General Yoav Mordechai said on his Facebook page.
In November, days after a mysterious explosion was reported near the city of Isfahan, Meridor himself told Israeli Army Radio: "There are countries who impose economic sanctions and there are countries who act in other ways in dealing with the Iranian nuclear threat."
Exclusive Analysis's John Cochrane noted that while there was no evidence of Israeli involvement "the Israelis don't seem to mind giving the impression that they may have been."
Some Middle East watchers such as former British diplomat Carne Ross thinks the one option that has not been tried seriously is simply talking to Iran about regional security.
Israeli concerns about Iran's nuclear program were understandable but the Iranians "have a covert war being waged against them...tension is mounting and conflict would be disastrous for everybody, so we have to examine alternatives."
"If they feel threatened the one way to address this is to talk about it with them," he said.
But other experts say the mistrust between Iran and Washington is so great that the prospects of contacts are poor.
Economic sanctions may be far more effective than any covert operation, some analysts say.
Prices of staples are soaring, the rial currency has plummeted and inflation is rising rapidly. Working class Iranians are under economic pressure. With the parliamentary elections in March, the first nationwide vote since 2009, the Iranian clerical establishment is worried that Iranians might stay away from the ballot boxes over economic dissatisfaction.
The last Iranian election was followed by eight months of violent protests. The authorities successfully put the uprising down through force, but since then the Arab Spring has shown the vulnerability of governments in the region to public anger fuelled by economic hardship.
Despite the mounting tension, Iranian leaders have to stick to the country's nuclear course, because otherwise they will risk losing their core hardline supporters, also essential to secure a high turnout in the March vote, analysts say.
"Iranians have always managed to cope with sanctions, but now with talks about oil embargo the authorities feel cornered. That is why they have increased the volume of harsh rhetoric," said Iranian analyst Khosro Karami.
"They will do anything to prevent street unrest, which will jeopardize the clerical establishment's existence."
Reporting by William Maclean; Additional reporting by Peter Apps in London, Dan Williams in Jerusalem and Fredrik Dahl in Vienna; Editing by Peter Graff