As more details surface about the suicide bomber who killed six people in an attack on Israeli tourists Wednesday, the Obama administration's refusal to blame Iran remains a mystery.   

Privately, U.S. officials insist Iran and Hezbollah are responsible for the bombing, which left 30 people injured at the airport of the Bulgarian vacation city of Burgas. Publicly, the White House remains tightlipped on the regime's involvement. 

On Thursday, U.S. officials began letting the cat out of the bag in interviews with The New York Times' Nicholas Kulish and Eric Schmitt:

One senior American official said the current American intelligence assessment was that the bomber ... had been “acting under broad guidance” to hit Israeli targets when opportunities presented themselves, and that the guidance had been given to Hezbollah, a Lebanese militant group, by Iran, its primary sponsor. Two other American officials confirmed that Hezbollah was behind the bombing, but declined to provide additional details. The attacks, the official said, were in retaliation for the assassinations of Iranian nuclear scientists, for which Iran has blamed Israeli agents.

The leaking of American intelligence didn't stop there. Today, multiple U.S. officials told Reuters' Mark Hosenball the same thing. 

U.S. officials say they increasingly concur with Israeli assessments that Iran and its proxies organized the killing of seven Israeli tourists in Burgas by a suicide bomber after they boarded an airport bus.

Surprisingly, the intelligence leaks aren't even confined to the federal government. In an impressive scoop, Hosenball even received "restricted" police documents from the New York Police Department pinning the blame on the Iranian-backed group.  

The documents, labeled "Law Enforcement Sensitive," said that this week's suicide bomb attack in Bulgaria was the second plot to be unmasked there this year ... Each plot was attributed to Iran or its Lebanese Hezbollah militant allies, said the reports, which were produced following the bombing in Burgas, Bulgaria of a bus carrying Israeli tourists.

Today, when asked about Iran's involvement in the attacks, White House press secretary Jay Carney declined to assign blame. “We don't have any confirmation yet. We are working to assess the facts and, with our partners, to discover who was responsible,” he told reporters. “It is certainly the case that Hezbollah and Iran have been bad actors, as a general matter. But we're not, at this point, in a position to make a statement about responsibility.” 

The reluctance to assign blame while a flurry of U.S. officials make unequivocal statements raises a number of intriguing questions. Is there a lack of hard evidence? Is this a bad time diplomatically to condemn the Iranian government? Or maybe the White House is simply exercising due caution?

One aspect that sticks out about the leaks to Reuters and The New York Times is the absence of any proof cited by the anonymous officials. While the Reuters report does cite evidence for previous Iranian-backed attacks (most notably, a plot in Azerbaijan carried out by the Iranian Revolutionary Guards), it doesn't provide evidence for the Bulgarian attack. Additionally, a report by the Christian Science Monitor's Nicholas Blanford raises skepticism about Hezbollah's involvement. "The attack by a suspected suicide bomber bore little resemblance to past bomb spectaculars pinned on Hezbollah," writes Blanford. "Both Iran and Hezbollah have denied any involvement in the attack."

It's also possible that the U.S. merely wants to avoid the appearance of jumping the gun on blaming Iran, something observers criticized Israel for on Wednesday after Prime Minister Benjamin Netanyahu blamed Iran just a few hours after the explosion went off.  “Iran is the one behind the wave of terror. Iran is the No. 1 exporter of terror in the world,” he said at a news conference. For its part, Bulgaria is also sitting pretty. “We’re not pointing the finger in any direction until we know what happened and complete our investigation,” Foreign Minister Nickolay Mladenov said. Regardless, you can expect Israeli diplomats to press Washington for a clear condemnation of Iran in the coming days. As Netanyahu said yesterday, “The time has come for all countries that know the truth to speak it."