When 32-year-old Iranian nuclear scientist Mostafa Ahmadi-Roshan was assassinated in a car bomb attack Wednesday, Iran immediately accused Israel of carrying out the attack. At this point, it's increasingly difficult to argue to the contrary.

Without question, a number of aspects of yesterday's attack remain mysterious and warrant a skeptical review of Iran's account, which it is taking to the United Nations. For instance, how were no culprits found when the alleged motorcyclist planted a magnetic bomb under a scientist's car in broad daylight with several witnesses present? This isn't even the first time such an attack has happened amid busy, rush-hour traffic. Another thing to keep in mind: Israel isn't the only state opposed to Iran's nuclear advancement: Sunni Arab governments could have also waged the attack. And finally, some of Iran's nuclear scientists who have been assassinated had ties to opposition groups and may have been targeted by Iran's own government. The preponderance of evidence, however, suggests Israel carried out the attack, with the possible assistance of the U.S. Here's why:

A former Israeli official admits to the assassination campaign Speaking with The New York Times, a "former senior Israeli security official," tells the newspaper the assassination campaign is "part of a larger Israeli strategy to prevent all-out war." Speaking anonymously, he says “I think the cocktail of diplomacy, of sanctions, of covert activity might bring us something,” the former official said. “I think it’s the right policy while we still have time.”

Israel doesn't deny it Of course, nation states shouldn't be required to respond to every allegation but when there's a high plausibility like this, Israel's refusal to comment is a response in itself. Speaking on condition of anonymity, an Israeli official tells The Washington Post that "It is not our policy to comment on this sort of speculation when it periodically arises." By contrast, the U.S. has outspokenly denied any involvement in the attack. “The United States had absolutely nothing to do with this,” said Tommy Vietor, a spokesman for the National Security Council. In an odd move, raising more questions than answers, Israel's chief military spokesman Brig. Gen. Yoav Mordechai wrote on his Facebook page "Don’t know who settled the score with the Iranian scientist, but for sure I am not shedding a tear.” According to The Post, the comment "sparked a debate on his page, with some readers saying he should be more discreet."

Past comments In the run-up to the assassination, some of Israel's public comments also raise suspicions. The National Post's Peter Goodspeed notes, "On Tuesday, Lieutenant-General Benny Gantz, Israel’s military chief of staff, told a special parliamentary committee Iran should expect more 'unnatural' events in 2012." Additionally, "Last year, Dan Meridor, Israel’s Minister for Intelligence & Atomic Matters, told Israeli Radio, 'There are countries that impose economic sanctions and there are countries who act in other ways.'" 

Israel's history As the Times' Scott Shane points out "Israel has used assassination as a tool of statecraft since its creation in 1948, historians say, killing dozens of Palestinian and other militants and a small number of foreign scientists, military officials or people accused of being Holocaust collaborators." 
 
Expert opinion If you're not one for looking at the matter yourself, there's always the experts, scholars and think tankers. According to The Times, "experts believe" the campaign was carried out by Israel. Speaking to the paper, Patrick Clawson, director of the Iran Security Initiative at the Washington Institute for Near East Policy, says “I often get asked when Israel might attack Iran,” he said. “I say, ‘Two years ago.’” He adds: “Sabotage and assassination is the way to go, if you can do it. It doesn’t provoke a nationalist reaction in Iran, which could strengthen the regime. And it allows Iran to climb down if it decides the cost of pursuing a nuclear weapon is too high.”