American intelligence agencies had indications three days beforehand that the Syrian regime was poised to launch a lethal chemical attack that killed more than a thousand people and has set the stage for a possible U.S. military strike on Syria.

The disclosure -- part of a larger U.S. intelligence briefing on Syria's chemical attacks -- raises all sorts of uncomfortable questions for the American government. First and foremost: What, if anything, did it do to notify the Syrian opposition of the pending attack?

In a call with reporters Friday afternoon, senior administration officials did not address whether this information was shared with rebel groups in advance of the attack. A White House spokeswoman declined to comment on whether the information had been shared.

But at least some members of the Syrian opposition are already lashing out at the U.S. government for not acting ahead of time to prevent the worst chemical attack in a quarter-century. "If you knew, why did you take no action?" asked Dlshad Othman, a Syrian activist and secure-communications expert who has recently relocated to the United States. He added that none of his contacts had any sort of prior warning about the nerve gas assault -- although such an attack was always a constant fear.

Razan Zaitouneh, an opposition activist in the town of Douma, one of the towns hit in the Aug. 21 attack, said she had no early indication of a major chemical attack. "Even the moment [the attack hit], we thought it was as usual, limited and not strong," she told The Cable in an instant message. That only changed when "we started to hear about the number of injuries."

"It's unbelievable that they did nothing to warn people or try to stop the regime before the crime," Zaitouneh added.

The U.S. intelligence community is now all but certain that the Syrian regime used chemical weapons on rebels and civilians in the suburbs of Damascus nine days ago. And part of that certainty were the military's signs of advance preparation for an attack.

"In the three days prior to the attack, we collected streams of human, signals and geospatial intelligence that reveal regime activities that we assess were associated with preparations for a chemical weapons attack," said a U.S. intelligence report the Obama administration released Friday.

"Multiple streams of intelligence indicate that the regime executed a rocket and artillery attack against the Damascus suburbs in the early hours of August 21," the report added. Satellites detected that the weapons were launched from territories held by the regime. They landed in rebel-controlled or contested neighborhoods.

The intelligence assessment is based on "a substantial body of information," including satellite imagery, intercepted communications, and social media reports from the scene of the attack.

"Our high confidence assessment is the strongest position that the U.S. Intelligence Community can take short of confirmation," the report said. "We will continue to seek additional information to close gaps in our understanding of what took place."

There had been reports of chemical attacks before the August 21 assault in Damascus. But it provided a wealth of new intelligence picked up by U.S. spy agencies that helped make the U.S. case for Syrian government culpability.

The Cable reported Tuesday that U.S. intelligence had intercepted a panicked phone call between an official at the Syrian Ministry of Defense and a leader of a chemical weapons unit in the hours after the attack. The minister demanded answers for the strike, which used a nerve agent. Those conversations helped convince U.S. officials that the Syrian regime was responsible.

The new intelligence assessment doesn't definitively answer whether the attack was ordered by the highest ranks of the government or if it was the work of a rogue military officer. But remarks this afternoon by Secretary of State John Kerry made it clear that the Obama administration is holding Syrian President Bashar al-Assad responsible.

"Read for yourselves the verdict reached by our intelligence community about the chemical weapons attack the Assad regime inflicted," Kerry said in remarks at the State Department.

The U.S. has determined that 1,429 people were killed in the attack, including at least 426 children. That number closely matches the casualty estimates reported by a Syrian opposition group yesterday. A separate report from the British Joint Intelligence Committee put the death toll much lower, at least 350 people. The U.S. assessment said the final tally "will certainly evolve as we obtain more information."

In releasing the intelligence report, the Obama administration sought to assure Americans that its conclusions were based on multiple verifiable sources, including public accounts, and that the intelligence community had not repeated the mistakes of 2003, when it incorrectly judged that Iraq possessed chemical weapons.

"We will not repeat that moment," Kerry said, emphasizing that the intelligence about the Syrian attacks had been vetted and reviewed.

In addition to U.S. satellite and signals intelligence, the report also relies on "thousands of social media reports" in the hours after the attack, noting they were sent from "at least 12 different locations in the Damascus area." Kerry mentioned the volume of the reports, as well. Ninety minutes after the attack, "all hell broke loose in the social media," Kerry said, noting that the reports conveyed images and video of victims of the attack, showing some of them dazed, twitching, foaming at the mouth, or dead.

The report said U.S. intelligence "identified one hundred videos attributed to the attack, many of which show large numbers of bodies exhibiting physical signs consistent with, but not unique to, nerve agent exposure."

Senior administration officials acknowledged that they had not yet obtained soil samples from the site of the attack to test for evidence of chemical agents. Physical evidence also wasn't part of the new assessment, an indication that the Obama administration believes the abundance of reporting from other sources is sufficient to make its case that the Syrian regime used chemical weapons.

The intelligence report also suggests a possible motive for the attack.

"We assess that the Syrian regime has used chemical weapons over the last year primarily to gain the upper hand or break a stalemate in areas where it has struggled to seize and hold strategically valuable territory. In this regard, we continue to judge that the Syrian regime views chemical weapons as one of many tools in its arsenal, including air power and ballistic missiles, which they indiscriminately use against the opposition."  

Kerry couched a U.S. response to the attacks in moral and humanitarian terms. But he did not advance any legal argument to support U.S. military action.

"2 things we did not hear from Secretary Kerry. (1) What is our military objective? (2) What legal justification is the Administration using?" Rep. Buck McKeon, the chairman of the House Armed Services Committee, tweeted after the secretary's remarks.

Meanwhile, the world is bracing for an anticipated attack on Syria. U.N. Secretary-General Ban Ki-moon told the United States and the four other big U.N. powers in a closed-door briefing today that it would take up to two weeks to determine whether chemical weapons have been used in Syria, according to diplomats briefed on the meeting.

Ban said that his chief U.N. weapons inspector, the Swedish scientist Ake Sellstrom, had initially insisted he would need three to four weeks to analyze samples collected at the site of the Aug. 21 attack. But Ban told the gathering, which included Samantha Power, the U.S. ambassador to the United Nations, and representatives from Britain, China, France, and Russia, that he had convinced Sellstrom to conclude his analysis in 10 to 14 days.

Ban's spokesman, Martin Nesirky, told reporters today that the U.N. could not immediately provide the Security Council with any findings on the nature of the substance that asphyxiated large numbers of people in the attack. Once the analysis is completed, he said, Ban will provide a report to the 15-nation council.

"We have to be very clear here that before the mission can draw any conclusion about this incident, the evaluation of all available information, including the laboratory analysis of all samples, must be completed," Nesirky said. "The team is doing its utmost to expedite the process of analysis."

Despite White House assertions that the U.N. inspectors' work is "redundant," Ban told the big powers that the U.N. would return to Syria in the future to resume inspections. Ban said that the inspectors had concluded their field inspections in the Damascus suburbs and that U.N. inspectors had visited a military hospital in Damascus to examine government claims that Syrian forces had been exposed to nerve agent during three recent chemical weapons attacks launched by rebel groups. Nesirky said that the chemical weapons team's translators had already left the country and that the technical experts were packing their bags, with plans to depart Syria Saturday morning.

Ban plans to meet Saturday with Angela Kane, the U.N. high representative for disarmament affairs, who is returning from Damascus, where she had negotiated access to the attack sites with Syrian authorities. But diplomatic sources said he has no plans to brief the Security Council over the U.S. Labor Day weekend.

In the meantime, some within the Syrian opposition are worried that any delays could give Assad time to bolster his defenses.

"This is one worry that we have. Since the international community has begun talking about a response to the chemical massacre, what we have noticed is that the Assad regime has started moving different military units into different areas," Khaled Saleh, the media director of the Syrian National Coalition and a member of the Syrian National Council, told The Cable. "So they're using that time to hide their more well-armed units. And you know, when they move them to schools, the U.S. and the international community can't do a whole lot about that."

If the U.S. doesn't strike Assad hard enough or if the strike is too limited, he will likely hit back at Syrians in response. "Our worry is that Assad will turn to Syrians and kill more of them," Saleh added.