In this Tuesday March 19, 2013, citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, black smoke rises from a building due to Syrian government forces shelling, in Aleppo, Syria. / AMC,AP Photo/Aleppo Media Center
Updated 5:51 PM ET
WASHINGTON A senior U.S. official is saying that the U.S. now has strong indications that chemical weapons were not used in the attack in Syria Tuesday.
Officials won't entirely rule out the possibility. But this official says additional intelligence-gathering has led the U.S. to believe more strongly that it was not a weaponized chemical attack.
The U.S. and allies have been looking into allegations by the Syrian regime that rebels carried out a chemical weapons attack in northern Aleppo province. Rebels blame regime forces.
There are fears President Bashar Assad would use chemical weapons against his people in the ongoing civil war and also concerns that al Qaeda linked rebels might obtain and use them.
The official wasn't authorized to speak publicly about the matter and spoke on condition of anonymity.
Earlier, some lawmakers on Capitol Hill wasted no time in calling for the Obama administration to consider military action. However, the drumbeat died down as the administration stressed the need for "facts" and real evidence before deciding how to proceed - a contrast to how the Bush administration reacted 10 years ago after using flawed intelligence about weapons of mass destruction as the impetus for the war in Iraq.
The hawkish reaction from members of Congress followed an intelligence briefing given to lawmakers where, CBS News' David Martin reports, one high level official called the use of chemical weapons in Syria "probable," going further than the administration's original assessment that there was a "potential" use of chemical weapons.
"There's, at least, a high probability they have used, either in -- most recently or in the past, some amount of chemical weapons," House Intelligence Committee chairman Rep. Mike Rogers, R-Mich., said on "CBS This Morning" Wednesday. "This is the time to act. Don't wait until we have 5,000 dead. That's too late." Senate Intelligence Committee chairman Dianne Feinstein, D-Calif., echoed Rogers' remarks Tuesday afternoon.
By Wednesday afternoon, however, after President Obama and others weighed in, the call for action quieted down, and the claim that the "high probability" that chemical weapons were used, was overshadowed by the administration's measured response to the allegations.
The president, during a news conference in Jerusalem with Israeli Prime Minister Benjamin Netanyahu Wednesday, stressed that before any action will be taken, his "teams" will investigate the allegations in Syria.
"In Syria right now you've got a war zone. You have information that's filtered out, but we have to make sure that we know exactly what happened, what was the nature of the incident, what can we document and what can we prove," said Mr. Obama, who has previously stated that the Syrian government's use of chemical weapons would be a "red line" for possible U.S. military action.
"Once we establish the facts, I have made clear that the use of chemical weapons is a game changer," he added.
Meanwhile, the United Nations will investigate the possible use of chemical weapons in Syria, which would amount to a crime against humanity, Secretary-General Ban Ki-moon announced Thursday.
The investigation could be broader than the Syrian government's request for an independent probe of a purported chemical weapons attack on Tuesday. Ban said he was aware of allegations of other, similar attacks and hoped the probe would ultimately help secure Syria's chemical weapons stockpile.
U.N. spokesman Martin Nesirky said France and Britain sent a letter to Ban Thursday asking for an investigation of three alleged chemical weapons attacks. He said Ban will review this suggestion as the U.N. develops the mandate for the investigation.
Syria is widely believed to have a large stockpile of chemical weapons. The government has not confirmed it, saying only that it would never use chemical weapons against its own people.
"My announcement should serve as an unequivocal reminder that the use of chemical weapons is a crime against humanity," the secretary-general said. "The international community needs full assurance that chemical weapons stockpiles are verifiably safeguarded."