BEIRUT, Lebanon -- Thousands of sick and dying Syrians had flooded the hospitals in the Damascus suburbs before dawn, hours after the first rockets landed, their bodies convulsing and mouths foaming. Their vision was blurry and many could not breathe.
Overwhelmed doctors worked frantically, jabbing their patients with injections of their only antidote, atropine, hoping to beat back the assault on the nervous system waged by suspected chemical agents. In just a few hours, as the patients poured in, the atropine ran out.
To avoid contamination, medics stripped new arrivals down to their underwear and doused them with water before taking them inside.
New patients kept coming. One doctor from the town of Kafr Batna likened the scene to a horror movie, with cars bringing in entire families -- fathers, mothers and children -- all of them dead.
The doctors soon faced a new problem: where to put the dead. Some were covered with blocks of ice to fend off the summer heat, others were wrapped in white sheets and lined up in rows so family members could identify the victims.
It would be hours before officials in Washington woke up on Wednesday to learn the extent of the massacre. President Obama, who had recently returned from a weeklong vacation and planned a quiet day at the White House before departing for a two-day bus tour across New York and Pennsylvania, was told of the attack in the Oval Office that morning during his regular intelligence briefing.
The White House issued a cautious public statement about the attacks from a deputy spokesman shortly before noon, but behind the scenes the president and his national security team were grappling with the urgency and enormity of the event: the largest mass killing of the Syrian civil war, and most likely the deadliest chemical weapons attack since Saddam Hussein's troops killed thousands of Kurds with sarin gas during the waning days of the Iran-Iraq war in 1988.
Interviews with more than two dozen activists, rebels and doctors in areas near the attack sites, as well as an examination of more than 100 videos and photos of the aftermath, back up this assertion.
Not only has the attack brought widespread condemnation on President Bashar al-Assad's government in Syria, which the United States and its allies are convinced carried out the strike, it is also already shaping up as an inflection point for a war that has been grinding on for more than two years and has claimed more than 100,000 victims.
An American president who has tried desperately to keep the United States out of another war in the Middle East is now weighing a military attack on Syria -- cornered by his own statement that a large-scale chemical weapons strike would be a "red line" forcing Washington to respond.
Mr. Assad's government has repeatedly denied using chemical weapons, while blaming rebels for reported attacks. But Western nations say they have solid evidence that the Syrian government has used such weapons on at least two occasions before last Wednesday. And the supplies of atropine on hand in rebel-held areas just outside Syria's capital testify to the repeated, if limited, use of chemical agents as a tactical weapon in what has become a street-by-street war of attrition, the rebels and doctors said.
If the United States does get involved, it will most likely be because of the scale of what took place in the dead of night last Wednesday, in towns just outside Damascus that the government was determined to retake. The attacks caused such chaos among residents that the death toll is still unknown, and many are still uncertain about the fate of their relatives.
"Those are my cousins," said one person in a video shot in the city of Hamouriyeh, pointing to the ground where the bodies of a man and his two children lay.
"I'm still looking for the rest," he said. "Five or six of them."
By nightfall in Syria, the bodies that were unclaimed had been buried in an archipelago of new mass graves. Before laying them to rest, activists put numbers on their foreheads and snapped photos -- in case their families came looking for them later.
Many Trapped at Home
It began just after 2 a.m.
Those who heard the explosions and lived to tell about them were surprised at the sound, saying it was "like a water tank bursting" or "like opening a Pepsi bottle."
Then came the smell, which burned eyes and throats, like onions or chlorine.
The effects were immediate and devastating.
At the time of the strikes, a few hours before morning prayers, most people were still asleep in their homes. The substance released by the barrage of rockets, which crashed into suburbs on two sides of Damascus, killed many people before they were even able to get out of their beds.
The deadliest of the attacks struck at the heart of a region known as Eastern Ghouta, an area northeast of Damascus whose towns have swelled into cities in recent decades with an influx of mostly poor Sunni Muslims from the countryside, the key constituency of the anti-Assad uprising.
Towns in the area have been held for more than a year by various factions of the rebellion. Unlike in northern and eastern Syria, extremist groups like the Nusra Front are not dominant. The area's economic isolation made it fertile ground for the rebellion, and it has proved to be a perpetual threat to Mr. Assad's control over the capital region.
The neighborhoods are dotted with homes that have been damaged -- or have collapsed outright -- from the persistent government shelling over the past year.
In the months before Wednesday's attack, according to interviews with rebels, the battle around Eastern Ghouta had reached a stalemate. While both sides frequently carried out guerrilla raids and sniper attacks, the front lines had moved little.
In the meantime, the government had sought to break the stalemate by severing the region's links to the capital and starving rebel troops in Eastern Ghouta. Shipments of flour, fuel and electricity to the area were stopped, and government troops on the few remaining byways confiscated bread and siphoned fuel from gas tanks to ensure it did not reach the rebels.
Shortly after Wednesday's rocket barrage began, rebel fighters spread the news of the assault by shouting, "Chemical attack!" into their walkie-talkies while loudspeakers affixed to minarets on the top of mosques blared warnings to residents to flee or to seek fresh air on their rooftops.
As in many rebel areas, residents had grown used to dealing with government attacks, instincts that this time only increased the death toll.
According to local doctors, some people took cover in basements, where the gas settled and suffocated them. Medics and photographers who had become accustomed to rushing to the site of attacks arrived too quickly, succumbing to the gases themselves.
The attacks appeared to fit into a pattern of continued escalation by government forces throughout the war, with large strikes on residential areas that appear to serve no immediate tactical purpose.
Such attacks seem to be aimed not at killing rebel fighters, but at terrifying the rebels' civilian backers in strategic areas that Mr. Assad's forces have been unable to subdue.
"They knew that people's sons were on the front lines, so if you hit their families, they would go back and check on them and it would be easier to invade," said an activist from Zamalka who gave only his first name, Firas. But he said that the tactic had not worked and had instead rallied rebel fighters to defend their positions.
Some military analysts said that the apparent chemical attack appeared to be part of a broader operation by Mr. Assad's forces, which have also used tanks, conventionally armed rockets and air power to wrest control of rebel areas around the Syrian capital.
"It appears that they were trying to break resistance in the Damascus area, which they have been trying to do unsuccessfully for some time," said Jeffrey White, a former Middle East analyst with the Defense Intelligence Agency who is now a fellow at the Washington Institute for Near East Policy.
Firas, the activist, said he was driving home with some friends when he heard about the attack over his walkie-talkie. He said he was terrified, since no one knew where the attacks had occurred and how far the suspected gas had spread. They used wet pieces of cloth to cover their noses and mouths and sped out of town to a field hospital farther east.
Whatever the chemicals used, the carnage caused by Wednesday's attack overwhelmed field hospitals on the outskirts of Damascus. Bodies covered tile floors, stretched down hallways and were laid out on sidewalks and streets.
A doctor from the town of Kafr Batna said he rushed to his clinic soon after the attack and found 100 patients.
"We had men, women and children, all of them choking and having trouble breathing," said the doctor, Sakhr. "Some of them had foam coming out of their mouths and nostrils and many had lost consciousness."
The medical charity Doctors Without Borders said Saturday that three clinics it supports in the area recorded 355 deaths.
The Syrian Observatory for Human Rights, which tracks the conflict from Britain through a network of contacts inside Syria, said it had confirmed the deaths of 322 people, including 54 children and 82 women.
Some activists have compiled lists of names and say that more than 1,000 people were killed in Wednesday's predawn attack.
By the end of that day, Dr. Sakhr of Kafr Batna said, 16 of the 160 bodies collected at his clinic had not been claimed. Volunteers took the bodies to a nearby graveyard, photographed their faces one by one, and buried them in a mass grave.
For those who survived, there was a different kind of grim reckoning.
Nearly a week after the attack, Dr. Sakhr said local residents who had not fled the area were flooding him with questions about where to sleep to protect themselves from future attacks.
Others were still searching for lost relatives, including children who had been taken in by strangers after their parents disappeared.
"Some found their relatives, said 'praise God' and sat down next to them," said Dr. Sakhr.
"Others didn't find them, and had to look elsewhere."
A Careful Response
As video images and eyewitness accounts of the assault began to spread through social media, President Obama was easing back into his routine after a week away on Martha's Vineyard. His only public event last Tuesday, hours before the attack began, was welcoming players from the undefeated 1972 Miami Dolphins to the White House for a much-belated ceremony.
The president had planned no public events for Wednesday. When he learned of the attack during his intelligence briefing that morning, many hours after it had occurred, American intelligence about the attack was still sketchy. But officials said that if the reports of chemical weapons use proved to be true, the attack was on a far greater scale than chemical attacks earlier this year.
Still, the White House moved carefully, driven in part by its experience with smaller-scale chemical weapons attacks in Aleppo and on the outskirts of Damascus. In those cases, a senior official said, there was conflicting evidence long afterward.
"In the past, it took weeks to match eyewitness accounts with intelligence," said a senior official, speaking on the condition of anonymity to discuss the internal deliberations. "This time, there was a consistency in all the information that was coming in for the first 24 to 48 hours."
In the subsequent statement, which was only two paragraphs, the White House urged the Assad government to allow United Nations investigators to visit the site and put the emphasis on gathering more information. The statement was issued by the White House's deputy spokesman, Josh Earnest, who declined to speculate about who was culpable for the attack.
Mr. Obama kept his travel plans to upstate New York on Thursday, although as his armored bus rolled from Buffalo to Rochester to Seneca Falls, he was on the phone several times with his national security adviser, Susan E. Rice.
At the White House that morning, Ms. Rice had convened a three-hour meeting of cabinet officials, including Secretary of State John Kerry, Defense Secretary Chuck Hagel and John O. Brennan, the director of the C.I.A. Military officials from United States Central Command joined the meeting by video.
The debate was robust, officials said. Some officials argued forcefully for military action, while others raised potential dangers about American missile strikes, including fears that they would destabilize the region and set off a vast new refugee flow into Turkey, Jordan and Lebanon.
One question that puzzled intelligence officials was exactly what kind of chemicals were used in the attack. American spy agencies conferred with allied intelligence services in Europe, Israel and Arab countries to get a clearer picture of what happened. In Israel, three Israeli officials briefed on the attack said they believed the rockets carried a "cocktail" of sarin gas mixed with several other components. Syria's government is believed to have large quantities of sarin, mustard gas and VX.
One Israeli official even suggested that whoever planned the rocket barrage might have miscalculated.
"It's quite likely that there was kind of an operational mistake here," one senior Israeli suggested. "I don't think they wanted to kill so many people, especially so many children. Maybe they were trying to hit one place or to get one effect and they got a much greater effect than they thought."
Meanwhile, Mr. Kerry made phone calls to foreign ministers from Europe and the Arab world, hoping to build international support for a potential military strike against Mr. Assad.
On the day of the White House meeting, he called Syria's foreign minister, Walid al-Moallem, to complain that the Assad government had not allowed United Nations inspectors to quickly visit the sites of the suspected attack.
It was a rare high-level contact between American and Syrian officials in the time since the United States closed its embassy in Damascus last year.
As Thursday wore into Friday, American officials said, it became clear that the Assad government was still thwarting the members of the United Nations team -- who had arrived in Damascus days earlier to inspect other possible chemical weapons sites -- from visiting the scenes of Wednesday's attack. Russia, long a supporter of President Assad, was blaming rebel forces.
Russia's public statements blaming the rebels hardened the views of White House officials. By the time the full National Security Council assembled, with Mr. Obama presiding, on Saturday morning, "the focus had really shifted to how we respond to this event, not whether we respond," a senior official said.
Ben Hubbard reported from Beirut, and Mark Mazzetti and Mark Landler from Washington. Jodi Rudoren contributed reporting from Jerusalem, Michael R. Gordon from Washington and Hwaida Saad and Mohammed Ghannam from Beirut.
This article originally appeared in The New York Times.