* Iran's top nuclear scientist killed in attack on Nov. 27
* Killing followed other significant security lapses
* Iran blames attack on Israel, Israel has not commented
By Parisa Hafezi
DUBAI, Dec 1 (Reuters) - The killing of Iran's top nuclear scientist has exposed security gaps which suggest its security forces may have been infiltrated and that the Islamic Republic is vulnerable to further attacks.
Mohsen Fakhrizadeh's killing on Friday followed two other big security lapses -- the theft of Iran's nuclear archive and a fire at a nuclear facility this year that some Iranian officials blamed on cyber sabotage.
With Fakhrizadeh the fifth Iranian nuclear scientist killed in targeted attacks since 2010, security experts are suggesting Iran's enemies have found its Achilles' heel.
Commander Hossein Dehghan, a former defence minister, told state television Fakhrizadeh had been killed because of "infiltration into Iran's security structure."
Iran's elite Revolutionary Guards' Security Force, or Close Protection Unit, assigns bodyguards and security officers to top military and civilian officials.
"His assassins obviously operated based on detailed intelligence about martyr Fakhrizadeh's movements," an Iranian security official told Reuters after Fakhrizadeh was killed in an attack on his car on a highway near Tehran in broad daylight.
"It is clearly a security weakness and many questions should be answered," said the official, who asked not to be named because of the sensitivity of the matter. "We should know whether there are spies among security people and locate the leak. This is essential for us."
Iran's military and clerical rulers have accused Israel of killing Fakhrizadeh. Israel had identified him as a prime player in what it says is Iran's nuclear weapons quest though Tehran denies seeking nuclear arms.
Israel has not commented on the incident but in the past has acknowledged pursuing covert operations against Iran's nuclear programme to gather intelligence.
"There are conflicting accounts on how Fakhrizadeh was assassinated, but a degree of infiltration is certain – and this is what worries the Islamic Republic the most," said Kasra Aarabi, an Iran analyst at the Tony Blair Institute for Change think tank.
Iran has given contradictory details of the killing of Fakhrizadeh. Shortly after he was killed, witnesses told state television that a truck exploded before a group of gunmen opened fire on Fakhrizadeh's car.
On Sunday, Ali Shamkhani, Secretary of the Supreme National Security Council of Iran, told state TV it was "a very sophisticated assassination that was carried out remotely with electronic devices" and with no people on the ground.
"We have no idea whether these reports of a satellite-controlled-truck-mounted-machine-gun are true, but they serve two key purposes for Tehran: trying to deflect embarrassment by portraying the assassination as remarkably sophisticated; and showing how quickly they can crack the case," said Henry Rome, senior analyst with Eurasia Group.
Iran's inability to thwart such operations could encourage future attempts.
"The existence of large chinks in Iran's nuclear armour is sure to raise questions over the role sabotage could or should play in any future counter proliferation policy towards Tehran," said Behnam Ben Taleblu, senior fellow at the Foundation for Defense of Democracies (FDD) think tank in Washington.
In 2018, Israeli Prime Minister Benjamin Netanyahu announced that Israel had smuggled hundreds of kilograms of paper and digital files on Iran's secret nuclear weapons programme out of the Islamic Republic. Iran rejected this as fraudulent.
Fakhrizadeh's killing comes months after a U.S. drone strike in Iraq killed Qassem Soleimani, head of the Revolutionary Guards' elite Quds Force. Tehran retaliated by launching missile strikes against U.S. targets in Iraq. In July, Iran executed an Iranian man convicted of spying for U.S. and Israeli intelligence. The Judiciary said Mahmoud Mousavi-Majd, who was arrested in 2018, had spied on Soleimani but that the case was not connected to Soleimani's killing on Jan. 3.
"CRUSHING" RESPONSE OR NOT?
Iran has said it will launch a "crushing" response to Fakhrizadeh's killing.
Political analysts said retaliatory measures could make it hard for U.S. President-elect Joe Biden, who takes office on Jan. 20, to scrap President Donald Trump's "maximum pressure" policy on Iran. Biden has said that if Tehran resumes compliance with the 2015 agreement with six powers limiting its nuclear programme, he will return to the pact, which Trump quit in 2018 and then reimposed sanctions that have hit Iran's economy hard.
"Iran will retaliate but it will not be a crushing response that could lead to full-blown war with Israel," said an analyst in Tehran, who asked not to be named.
President Hassan Rouhani has said Iran will retaliate for Fakhrizadeh's killing at "the proper time" but will not "fall into Israel's trap" in the last weeks of Trump's presidency.
"Considering that Trump is still in the White House for another two months, Tehran's escalation options are going to have to be restrained," said Taleblu. "Despite talk of a potential military reprisal against Israel directly, that would be a low-probability high-impact scenario."
(Editing by Timothy Heritage)
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