While the report's authors did not assign blame for the attack on the outskirts of Damascus, the details it documented included the large size and particular shape of the munitions and the precise direction from which two of them had been fired. Taken together, that information appeared to undercut arguments by President Bashar Assad of Syria that rebel forces, who are not known to possess such weapons or the training or ability to use them, had been responsible.
The report, commissioned by Secretary General Ban Ki-moon, was the first independent on-the-ground scientific inquest into the attack, which left hundreds of civilians gassed to death, including children, early on Aug. 21.
The repercussions have elevated the 30-month-old Syrian conflict into a global political crisis that is testing the limits of impunity over the use of chemical weapons. It could also lead to the first concerted action on the war at the U.N. Security Council, which up to now has been paralyzed over Syria policy.
"The report makes for chilling reading," Ban told a news conference after he briefed the Security Council. "The findings are beyond doubt and beyond the pale. This is a war crime."
Ban declined to ascribe blame, saying that responsibility was up to others, but he expressed hope that the attack would become a catalyst for a new diplomatic determination at the United Nations to resolve the Syrian conflict, which has left more than 100,000 people dead and millions displaced.
There was no immediate reaction to the report from the Syrian government. But just two days before the report was released, Syria officially agreed to join the international convention on banning chemical weapons, and the United States and Russia, which have repeatedly clashed over Syria, agreed on a plan to identify and purge those weapons from the country by the middle of next year. Syria has said it would abide by that plan.
The main point of the report was to establish whether chemical weapons had been used in the Aug. 21 attack in the Damascus suburb of Ghouta, an area long infiltrated by rebels. The U.N. inspectors concluded that "chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic, also against civilians, including children, on a relatively large scale."
The weapons inspectors, who visited Ghouta and left the country with large amounts of evidence on Aug. 31, said, "In particular, the environmental, chemical and medical samples we have collected provide clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used."
But the report's annexes, detailing what the authors found, were what caught the attention of nonproliferation experts.
In one particularly incriminating piece of information, the inspectors said the remnants of a warhead found in the attack's aftermath showed its capacity of sarin to be about 56 liters - far larger than the suspected delivery systems used in alleged chemical weapons attacks before the Aug. 21 strike.
The investigators were unable to examine all of the munitions used, but were able to find and measure several rockets or their components. Using standard field techniques for ordnance identification and crater analysis, they established that at least two types of rockets had been used, including an M14 artillery rocket bearing Cyrillic markings and a 330-millimeter rocket of unidentified provenance.
These findings, though not presented as evidence of responsibility, were likely to strengthen the argument of those who claim the Syrian government bears the blame, because the weapons in question had not been previously documented or reported to be in possession of the insurgency.
Moreover, those weapons are fired by large, conspicuous launchers. For rebels to have carried out the attack, they
would have had to organize an operation with weapons they are not known to have and of considerable scale, sophistication and secrecy - moving the launchers undetected into position in areas under strong government influence or control, keeping them in place unmolested for a sustained attack that would have generated extensive light and noise, and then successfully withdrawing them - all without being detected in any way.
One annex to the report also identified azimuths, or angular measurements, from where rockets had struck, back to their points of origin. When plotted and marked independently on maps by analysts from Human Rights Watch and by The New York Times, the U.N. data from two widely scattered impact sites pointed directly to a Syrian military complex.
Other nonproliferation experts said the U.N. report was damning in its implicit incrimination of Assad's side in the conflict, not only in the weaponry fragments but the azimuth data that indicated the attack's origins. An analysis of the report posted online by the Arms Control Association, a Washington-based advocacy group, said "the additional details and the perceived objectivity of the inspectors buttress the assignment of blame to Bashar al-Assad's Syrian government."
The United States and its allies seized on the volume of data in the report to reaffirm their conclusion that only Syrian government forces had the ability to carry out such a strike, calling it a validation of their own long-held assertions.
Both the British and U.S. ambassadors to the United Nations also told reporters that the report's lead author, Dr. Ake Sellstrom, a Swedish scientist who joined Ban in the Security Council briefing, had told members that quality of the sarin used in the attack was high.
"This was no cottage-industry use of chemical weapons," said the British ambassador, Sir Mark Lyall-Grant. "The type of munitions, the trajectories, which confirmed the analysis that British experts have done about the provenance of where the rockets were fired from, all of that confirms, in our view, that there is no remaining doubt that it was the regime that used chemical weapons."
Samantha Power, the U.S. ambassador to the United Nations, acknowledged implicitly the credibility issue that has confronted the United States on Syria chemical weapons use, a legacy of the flawed intelligence on weapons of mass destruction that led the United States into the Iraq war a decade ago.
"We understand some countries did not accept on faith that the samples of blood and hair that the United States received from people affected by the Aug. 21 attack contained sarin," she said. "But now Dr. Sellstrom's samples show the same thing. And it's very important to note that the regime possesses sarin, and we have no evidence that the opposition possesses sarin."
Russia's ambassador, Vitaly I. Churkin, said there were still too many unanswered questions. In talking to reporters, he asked, if the Syrian forces had indeed been responsible and sought to attack insurgents, "how is it possible to fire projectiles at your opponent and miss them all?"
"We need not jump to any conclusions," he said.
The report's release punctuated a tumultuous week spawned by the global outrage over the attack, in which an American threat of punitive force on the Syrian government was delayed as Russia proposed a diplomatic alternative and intense negotiations between the United States and Russia led to a sweeping agreement under which Syria's chemical weapons arsenal could be destroyed.
The United Nations, in danger of becoming irrelevant in helping to end the Syria conflict, was suddenly thrust back into a central role, with the Security Council now engaged in deliberations over an enforceable measure to hold Syria to its commitment on chemical weapons.
Secretary of State John Kerry and the foreign ministers of France and Britain said Monday that they would not tolerate delays in dismantling Syria's chemical weapons.
"It is extremely important that there are no evasions," William Hague, the British foreign secretary, said at a news conference with Kerry in Paris.
Kerry said, "If Assad fails in time to abide by the terms of this framework, make no mistake, we are all agreed - and that includes Russia - that there will be consequences."
The release of the report came as a separate panel of investigators from the U.N. Human Rights Council in Geneva said they were investigating 14 episodes of suspected chemical weapons use.