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Other sources in the intelligence community, who declined to comment on the record, suggested generally that the Obama administration may have been reluctant to aggressively respond to the previous incidents because they did not trigger the same level of global media attention generated by last month’s attack near Damascus.

The Obama administration has said more than 1,400 people, hundreds of them children, were killed in the attack, which was followed almost immediately by horrific videos, photos and witness accounts across social media facets of the Internet.

Some respected international organizations have concluded, however, that the prior incidents in Syria also involved the nerve agent sarin. In a posting on its website last week, Human Rights Watch said that “there is laboratory evidence that Sarin gas has been used in a previous attack in April on Jobar, near Damascus, when a photographer for Le Monde newspaper who was present at the time later tested for exposure to Sarin.”

The Obama administration was clearly aware of such reports. Roughly two months after the April incident, the White House released a statement attributed to deputy national security adviser Benjamin J. Rhodes, who said the U.S. intelligence agencies assess “that the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year.”

“The intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date; however, casualty data is likely incomplete,” Mr. Rhodes said in the June 13 statement. “The President has been clear that the use of chemical weapons — or the transfer of chemical weapons to terrorist groups — is a red line for the United States.”

As a result, according to the senior congressional source, the administration discretely began pushing forward with a policy of moving small American weaponry to Syria’s rebels.

Washington Times reporter Ashish Kumar Sen contributed to this article.