Iran urges UN to condemn nuclear scientists' killings

• Iran wants the UN and others to "condemn, in the strongest term" the "terrorist acts"
• Iran's U.N. ambassador said Israel is to blame for the attacks
• The Iraq-based MEK is blaming the Iranian government
• Three scientists have been killed in the last 2 years in similar attacks

Tehran, Iran (CNN) -- Iran has issued an impassioned letter to the United Nations Secretary General charging that the killings of Iranian nuclear scientists were terror attacks that followed a clear pattern -- an assertion the country's ambassador to the United Nations repeated to CNN in an interview Wednesday night.

"There is firm evidence that certain foreign quarters are behind such assassinations," the letter said, "It is highly expected from the Secretary-General of the United Nations, and President of the Security Council of the United Nations as well as all other relevant organs and bodies to condemn, in the strongest term, these inhumane terrorist acts."

Mostafa Ahmadi Roshan, a nuclear scientist, was killed in a blast in Tehran on Wednesday morning, the latest in a series of attacks against such scientists in recent years.

A motorcyclist placed a magnetic bomb under Roshan's Peugeot 405. Two other scientists were killed in a similar fashion in the capital city in the last two years.

Iran's U.N. Ambassador Mohammad Khazaee blamed Israel for the attacks during an interview with CNN Wednesday night.

"They are trying to assassinate the Iranian scientists to deprive Iranians from the right of using peaceful nuclear energy," Khazaee said. "We believe that these terrorist attacks are supported by some elements -- especially within the Israeli regime as well as some quarters around the world."

Other government officials in Iran leveled the same accusation throughout the day Wednesday, with Iranian lawmaker Kazem Jalali blaming the intelligence agencies of the United States and Israel.

The United Nations' nuclear watchdog, the International Atomic Energy Agency, also bore responsibility for passing on information about Iran's nuclear scientists to other countries, state-run IRNA news agency said.

On Wednesday, U.S. Secretary of State Hillary Clinton, rejected the claims.

"I want to categorically deny any United States involvement in any kind of act of violence inside Iran," she said.

"We believe there has to be an understanding between Iran, its neighbors and the international community that finds a way forward for it to end its provocative behavior, end its search for nuclear weapons and rejoin the international community and be a productive member of it."

Israel does not normally comment on such claims. However, Brig. Gen. Yoav Mordechai, a spokesman for the Israel Defense Forces, said on his Facebook page Wednesday: "I have no idea who targeted the Iranian scientist but I certainly don't shed a tear."

Roshan, 32, was a deputy director for commercial affairs at the Natanz uranium enrichment facility in Isfahan province and a graduate of Iran's Oil Industry University, according to the semi-official news agency Fars.

Natanz, which is said to have 8,000 centrifuges in operation, is one of two facilities that is enriching uranium in the country. This week, the IAEA identified the second in the mountains of Qom province.

Officials in the United States and other Western nations have ratcheted up sanctions against Tehran since a November report by the IAEA said the Iranian government was developing the technology needed to build a nuclear weapon. Last month, U.S. President Barack Obama announced sanctions against Iran's central bank.

Tehran maintains its nuclear program is for civilian energy purposes only. But the IAEA has said it cannot verify whether the intent of Tehran's nuclear program remains peaceful.

The attack also comes at a time when relations between Iran and the United States have rarely been as strained.

Iran sentenced Iranian-American and former Marine Amir Hekmati to death Tuesday for alleged espionage, prompting strong condemnation from the U.S. State Department.

Iran also aggravated tensions in the past month with its threat to close the strategically important Strait of Hormuz if Western nations carry through with sanctions on its oil industry to punish Tehran's lack of cooperation on its nuclear program.

In comments Wednesday, Russian Deputy Foreign Minister Sergei Ryabkov urged Western nations and Iran to avoid escalating the situation further, Russia's official Itar-Tass news agency reported.

The attacks have been eerily similar.

On January 12, 2010, Iranian university professor and nuclear scientist Massoud Ali Mohammadi died in a blast when an assailant stuck a bomb under his car. Majid Jamali Fashi, an Iranian, reportedly confessed to the bombing and was sentenced to death in August, IRNA reported at the time.

Prosecutors accused him of working for Israel's spy agency Mossad and said he was paid $120,000 by Israel to carry out the hit, Fars reported. Israel does not comment on such claims.

In November 2010, nuclear scientist Majid Shahriari was killed in a blast where, again, a bomb was stuck under a car by someone on a motorcycle. Another nuclear scientist, Prof. Fereydoun Abbasi Davani, and his wife were injured in a similar attack. Abbasi is now director of the Iran Atomic Energy Organization.

With no one claiming responsibility, the killings remain shrouded in mystery leaving experts only to speculate. Several said Israel could have had a hand in the attacks.

"The most likely contender among people who are following this is that the Israelis are doing it, possibly in cooperation with the Iranian mujahedin," said Trita Parsi, president of the National Iranian-American Council and author of the book, "A Single Roll of the Dice: Obama's Diplomacy with Iran."

"There's almost no downside for Israel," he said. The killings "take out nuclear assets and embarrass Iran" by showing that the regime can't prevent such attacks, Parsi said. And "if Iran retaliates with a violent act, then Israel can point to it as a reason to take military action against the regime."

A representative of the Mujahedin-e Khalq, or MEK, said the group had nothing to do with it and blamed the Iranian government. The Iraq-based group, also known as the People's Mujahedin, is dedicated to the overthrow of Iran's Islamic fundamentalist regime and classified by the U.S. government as an international terrorist group.

"The Iranian Resistance revealed last year that (Supreme Leader Ayatollah Seyed) Ali Khamenei ordered a special unit of the Revolutionary Guards that as soon as a sign is detected that one of the managers of experts of the nuclear program is distancing from the regime or has intention of leaving the country, he should be eliminated and the blame should be put on the (MEK)," said Shahin Gobadi, a spokesman for the National Council of Resistance of Iran, the political coalition that includes MEK.

Some have speculated that the Iranian regime itself could have been involved in at least one of the killings, the one in January 2010.

That attack came shortly after major riots against the regime, and many people thought the regime was behind that killing, Parsi said.

Mohammadi "did not seem to be a particularly valuable nuclear target," he said. Some reports suggested Mohammadi was an outspoken supporter of the "green movement," and had helped organize protests, Parsi said.

But the man killed in November 2010, Shahriari, and the one who survived an assassination attempt at the time, Davani, were a different story.

It "would make no sense for the Iranians to assassinate them," Parsi said. "They were critical nuclear assets."

CNN's Shirzad Bozorgmehr, Kevin Flower and Josh Levs contributed to this report.