One of the most precise and dramatic details cited by the Obama administration as proof that Syrian forces used chemical weapons in an Aug. 21 attack was the death toll, which an official U.S. government assessment put at 1,429 people, including 426 children.
The number, first released by the White House on Aug. 30, was underscored by Secretary of State John Kerry in a fiery indictment of Syrian President Bashar al-Assad, describing videos of what he said were victims of the attack, which Syria denies.
"Instead of being tucked safely in their beds at home, we saw rows of children lying side by side sprawled on a hospital floor, all of them dead from Assad's gas and surrounded by parents and grandparents who had suffered the same fate. The United States Government now knows that at least 1,429 Syrians were killed in this attack, including at least 426 children," he said.
Some U.S. congressional sources are now casting doubt on those figures.
Three congressional sources told Reuters that administration officials had indicated in private that some deaths might have been caused by the conventional bombing that followed the release of sarin gas in suburban Damascus neighborhoods. This disclosure undermined support for President Barack Obama's plan to strike Syria, they said.
A White House spokeswoman referred all questions about the death toll numbers - including a request for comment on whether controversy about the numbers was undermining support on Capitol Hill for administration policy - to intelligence agency spokespeople.
"The Intelligence Community has a high bar for its assessments but it is virtually impossible to achieve 100 percent certitude," said Shawn Turner, chief spokesman for the Office of the Director of National Intelligence. "That's not the way intelligence works.
"We are extremely rigorous in our methodology and we are constantly challenging ourselves to be more exacting," said Turner. "We have been thorough in our discussions with Congress about our methodology and I'm not aware of any concerns."
One of the congressional sources said that administration officials in closed door briefings said they could not rule out that some victims included in the U.S. death toll were killed either by conventional explosive parts of rockets which carried poison gas or in the artillery barrage the United States says followed the gas attack.
A second source, who is sympathetic to White House policy, said caveats administration officials attached to the 1,429 death total were of sufficient magnitude to cause the source to avoid citing the figure.
A third source said that administration officials confronted pointed questions from members of Congress about the accuracy of the numbers and acknowledged that they "couldn't be sure" about the cause of death for some people counted as victims of chemical poisoning.
An administration official familiar with the briefings denied that there had been any doubts as to how the 1,429 bodies were counted; a second official asserted that Capitol Hill officials had heard what they wanted to hear because so many legislators were opposed to Obama's plan.
Administration sources told Reuters that they relied on a valid intelligence methodology to make the death estimate. An official said that it involved analyzing video pictures of victims, then eliminating from the fatality total any live person, any dead body with visible injuries and shrouded bodies showing blood spots.
Classified intelligence tools then were used to confirm the provenance of the videos and to ensure that bodies were not counted twice, the official said. The official noted that U.S. intelligence had more resources to gather information than human rights or other non-governmental groups, which had smaller death tolls.
"Nobody who has looked at the intelligence thinks this number is way off," a senior U.S. official said.
"That's what the number was that day. We know 1,400 people were killed. As we get new information, the number could change," the senior U.S. official added.
French intelligence says deaths from the gas attacks could be as high as 1,500, but it reported confirmed deaths from video evidence of 281. Estimates of gas attack deaths by British intelligence, the London-based Syrian Observatory for Human Rights and non-governmental group Doctors without Borders fall within a range of 322 to 355.
The congressional sources said that some members of Congress asked to see raw intelligence gathered by U.S. agencies. But thus far, the administration has provided only reports summarizing intelligence from human informants, electronic eavesdropping and satellite images.
The Syrian government has denied launching any gas attack, although it has acknowledged it has such weapons and is in talks to give them up.
The United States first cited the 1,429 death toll in a four-page document released by the White House, calling it a "preliminary assessment." Administration officials said that estimate was based on intelligence analysis and never meant to be fixed in stone. Moreover, they expect the ultimate toll will be higher.
In recent days, the administration has avoided the precise figures of the early days.
On Sept. 9, White House National Security Advisor Susan Rice also rounded down the figures, saying that "more than 1400" were killed, including "more than 400 children."
In his speech to the nation on Tuesday night, Obama said that Assad's forces had "gassed to death over 1,000 people, including hundreds of children."
A White House official called it a "stylistic thing". "It's accurate and not meant to signal any walking away from the assessment's figure," the person said.
Paul Pillar, formerly the top Middle East expert for U.S. intelligence, told Reuters the United States should have rounded the figures from the start.
"The administration did not help its case by providing a number that misleadingly implied a degree of precision that would be nearly impossible to achieve amid a civil war," he said.