The Americans deny everything.
The Israelis also deny everything -- but with a smile, according to a senior U.S. official.
Regardless of who is killing Iran's nuclear scientists -- the Israelis, the Americans or the Iranians themselves -- there's no question that researchers and officials linked to Iran's alleged nuclear weapons program keep turning up dead.
Since 2007, four different scientists allegedly associated with the nation's nuclear weapons program have died via bomb, gunshot or poisoning, while a fifth barely survived a car bombing.
The most recent victim, 35-year-old Darioush Rezaeinejad, was shot in the neck outside his daughter's Tehran kindergarten on Saturday by two gunmen on a motorcycle. According to an unconfirmed report in an Israeli intelligence publication, Rezaeinejad was working on a nuclear detonator, and was seen daily at a nuclear lab in northern Tehran.
Rezaeinejad's murder sparked official outrage in Iran. State media published an online report Sunday evening in which a leader of the nation's parliament said the killing showed the "desperation" of the U.S. and Israel.
"When the Americans and the Zionist regime realized that they cannot stand against the resolve of the Iranian nation and [after they] witnessed our nuclear achievements they resorted to assassinating our scientists," said Kazem Jalali, head of the parliament's national security committee, according to the report.
In most cases, Iranian officials blame the deaths and disappearances on the West without equivocation. In November 2010, Fereydoon Abbasi, the head of Iran's Atomic Energy Organization was wounded when a motorcyclist detonated a magnetic bomb under their car by remote control. Abbasi was on a U.N. list of people sanctioned for suspected links to nuclear activities.
The same day, a magnetic bomb placed by a motorcyclist killed nuclear physicist Majid Shahriari in his car. President Mahmoud Ahmadinejad quickly blamed "Western governments and the Zionist regime" for the twin bombings.
Did Mossad Kill Ardeshir Hassanpour?
Sometimes, however, Iranian officials will downplay the deaths, as if casting doubt on the ability of hated foes to strike at will. On Sunday, the same state website that published the article decrying the murder of Darioush Rezaeinejad as a Western assault on Iran's nuclear program published a second story ten minutes later saying his death really wasn't such a big deal after all. Iranian intelligence minister Heidar Moslehi was quoted saying it was unclear who killed Darioush Rezaeinejad and that he wasn't a nuclear scientist anyway. "The assassinated student was not involved in nuclear projects and had no connection to the nuclear issue."
In early 2007, Ardeshir Hassanpour, an Iranian nuclear scientist working at a nuclear plant in Isfahan, died from what Iranian state media called "gas poisoning." While the private U.S. intelligence firm Statfor reported that Mossad had killed Hassanpour, the Iranian government said that Hassanpour was not involved in nuclear research and had died in an accident.
Iranian opposition groups, meanwhile, say that sometimes the Iranian government kills dissident scientists -- who may or may not have anything to do with nuclear weapons -- and then blames the murders on the West.
In January 2010, a motorcycle parked outside the house of nuclear physicist Masoud Ali Mohammadi was detonated by remote control when he walked past.
The regime blamed his murder on a "triangle of wickedness," meaning the U.S., Israel and their "hired agents." "Zionists did it," said Ahmadinejad. "They hate us and they don't want us to progress." Ali Larjani, chairman of the Iranian parliament, said the government had "clear information that the intelligence regime of the Zionist regime and the CIA wanted to implement terrorist acts."
But Western intelligence agencies had conflicting information about whether Mohammadi, a particle physicist, was really contributing to the nuclear program. An Iranian opposition group said Mohammadi had been killed by the regime because he was a supporter of Mir Hossein Mousavi, whom many believe actually won the 2009 Iranian presidential election before vote-tampering handed the victory to Ahmadinejad. A German-based opposition group released a photo of an alleged Arab hitman who had supposedly carried out Mohammadi's assassination on regime orders.
At Mohammadi's funeral, hundreds of regime loyalists waving anti-Israel banners packed the procession, where they clashed with supporters of Mousavi's Green Movement.
U.S. Official: Murder Charges 'Absurd'
A former senior intelligence official involved in efforts to thwart Iran's nuclear program told ABC News that assassinations of top Iranian scientists were usually assumed to be the work of Israel, but that the Israelis would never confirm or admit responsibility.
"Every time we ask," said the official, "they just smile and say, 'We have no idea what you are talking about.' "
The U.S., for its part, has officially denied any involvement in the deaths of Iranian scientists. A White House spokesman called the accusations "absurd" after Mohammadi's death. The CIA is known, however, to have recruited scientists as spies.
In 2007, after Iranian General Ali-Reza Asgari went missing in Turkey, the Iranian government said the intelligence official had been kidnapped by Mossad. The Israeli and Western media said he had defected, and was busy providing information on Iran's nuclear program.
Two years later, award-winning Iranian nuclear scientist Shahram Amiri disappeared while on a pilgrimage to Mecca. He later resurfaced in the United States after defecting to the CIA in return for a large sum of money, according to people briefed on the operation by intelligence officials. A spokesperson for the CIA declined official comment.
Amiri, a nuclear physicist in his early 30s, worked at Tehran's Malek Ashtar University, which is closely connected to Iran's Revolutionary Guard, according to the Associated Press.
When Amiri disappeared, Iran's Foreign Minister, Manouchehr Mottaki, and other Iranian officials blamed the U.S. for "kidnapping" Amiri.
After resurfacing in the U.S., Amiri released two videos, one in which he claimed he was kidnapped by the CIA, and another in which he denied he was kidnapped. He later repeated the kidnapping story publicly, said he had "escaped" from the CIA, and returned to Iran.
Amiri was recruited by the CIA because his relatively young age -- he was not 30 when he first began to work for the Americans -- and his proximity to more senior Iranian nuclear scientists would allow the CIA to collect intelligence on the program for many years, according to a U.S. official involved in the Amiri operation.
In January 2011, a dissident website reported that Amiri was being held in a Tehran prison and had been tortured.