NEW YORK – Ever since the White House unveiled a series of findings on Thursday that chemical weapons have been used in Syria’s civil war on at least two occasions, Western officials have been acknowledging that suspicions of their use took root in December.After claims first surfaced of a chemical attack on December 23 in Homs, one source described a strongly worded exchange between Assad regime officials and Russian diplomats, with the latter persuaded by Western powers that such an attack would broach a red line that, if crossed, would preclude even Russia from stopping an escalation of American involvement.The Jerusalem Post has learned that, in support of written pleas sent by Britain and France to UN Secretary-General Ban Ki- moon, US Ambassador to the UN Susan Rice met in private with Ban to issue a formal request from the Obama administration for an investigation into the use of chemical weapons in Syria, based on “credible evidence” acquired by US intelligence agencies.That meeting took place on March 20. The secretary-general announced an investigation the following morning.One former White House official said the intelligence roll-out reminded him of the nerves exhibited by the CIA in 2007, when concerned Israelis approached the Bush administration to discuss how to address a suspected nuclear facility under construction in eastern Syria.“All the intelligence agencies agreed that this was a nuclear reactor, but they couldn’t definitively prove it was part of a larger nuclear program,” the official said.Ever since Iraq, spooked US officials have qualified their intelligence assessments submitted to the Oval Office in tiers of certainty: low, medium or high confidence. How information earns higher confidence has become a matter of politics within the intelligence community.“They have one out: They can hope the amount of attention given to this [intelligence report] is sufficient to deter Assad from doing it again,” said Elliott Abrams, a former senior diplomat in the Reagan and George W. Bush administrations. “The question is: Is it really an intel problem, or is the administration trying to hide or politicize the intel?”On a conference call with reporters after the announcement, one White House official said the president sought to prove the “chain of custody” of the chemicals used – not just the fact that they were indeed released, but who released them, who made them and where they had come from.But without unfettered access to Syrian stockpiles as a point of reference, that chain of custody is impossible for investigators to prove, said Ray Zilinskas, director of the Biological Weapons Nonproliferation Program at Middlebury College’s Monterey Institute of International Studies in California.“The rebels don’t have the capabilities themselves to produce complex chemicals such as sarin,” which is what was found by the Americans, Zilinskas said. “But they do have the ability to acquire organophosphorus pesticides, which produces a lot of similar symptoms,” including miosis (excessive constriction of the pupils) and foaming at the mouth.Matthew Meselson, co-director of the Harvard Sussex Program on Chemical and Biological Weapons, agreed. He also noted that sarin has a shelf life of less than two years – unless very purely made – implying that the sarin used in the attacks was likely mixed by the Assad government after the civil war started in March 2011.To test for sarin, intelligence agencies would have to acquire either soil samples or samples of tissue, blood, urine or hair from humans or animals present during the chemical release. While the effects of less potent gases such as mustard would linger, sarin’s volatility makes it difficult to detect more than 10 days after an attack.Western intelligence officials therefore fear that an extensive, detailed report of findings would reveal their hand to the Assad government, possibly compromising agents or allies on the ground who were able to acquire evidence quickly.Speaking to the Post , Ambassador to Washington Michael Oren said intelligence work on Syria’s chemical weapons between Israel and the US – and in coordination with other Middle Eastern powers – is “intimate,” but that Israel’s strategic “red line” is different than the one oft-repeated by the American president since last August.“Prime Minister [Binyamin] Netanyahu has stated that Israel’s red line is any attempt by Syria to transfer chemical or other game-changing weapons to Hezbollah,” Oren said. “That is our red line, and we stand by it.“We are not pressuring, urging or even suggesting that the United States should take military action in Syria,” the ambassador continued. “All we have stated is that if a decision were to be made to provide weapons to rebels, those who receive them should be closely vetted.”Two days before the White House acknowledgment, and two days after meeting with US Secretary of Defense Chuck Hagel in Tel Aviv, Defense Minister Moshe Ya’alon openly cited instances of chemical weapons use in Syria.Hagel told journalists the announcement caught him by surprise and that the US was still searching for hard proof in “real intelligence.”“If the Israelis are going to force the president’s hand, they don’t want to waste it on this,” said Daniel Byman, director of research at the Saban Center for Middle East Policy at the Brookings Institution. “But certainly, the Israelis put this back on the agenda.“It’s rare that you get the perfect intelligence,” Byman added, “or the smoking gun that you need, as we saw painfully in Iraq.”Facing pressure from both sides of the aisle in Congress to match his words with deeds, US President Barack Obama reiterated his stance on Friday after meeting with King Abdullah II of Jordan: Chemical weapons were a “game changer,” and the US would have to respond – in one way or another.Foreign affairs experts, ranking congressmen and Senate Intelligence Committee members have all agreed: strategically, the administration has given itself little choice.“It’s not reasonable to seek perfect certainty – that becomes an excuse for inaction.And that worries me not about Syria, but about Iran,” Abrams said. “If we do nothing, the Iranians are going to draw the conclusion that this language means nothing.”Western officials believed in January that Syrian President Bashar Assad “got the message” after feeling sincere international pressure, Ron Prosor, Israel’s ambassador to the UN, said at the time.But that diplomatic success proved short-lived, as reports surfaced less than two months later of additional small-scale chemical attacks.