Syria's opposition accused President Bashar Assad's government of a chemical attack on Khan Sheikhoun, a town in the rebel-held province of Idlib.
Social media spread gruesome images of the Tuesday attack, which left more than 70 people dead and apparently involved a nerve gas, possibly sarin.
Syria's government, however, insisted that no such weapons had been deployed.
The attack spurred worldwide outrage against the Syrian government, renewing calls for Assad's removal from power.
Here's some more of what we know:
The timing of the attack
Activists reported the attack occurred around 6:30 or 7 in the morning local time Tuesday. Pro-opposition groups such as the Edlib Media Center published photos some hours later depicting corpses of children they said had suffocated to death because of toxic gases.
One opposition activist, Ahmad Sheikho, said in a WhatsApp conversation Thursday that he had seen the airstrike in the morning.
"I took pictures of the strike ... it was less than 100 yards from my house," he said.
U.S. radar spotted Syrian aircraft in the area around Khan Sheikhoun about 7 a.m. local time Tuesday, according to U.S. officials who requested anonymity because they weren't authorized to speak on the intelligence.
Both Russia and Syria initially denied their forces had flown in the vicinity of Khan Sheikhoun that day. But Thursday, Syrian Foreign Minister Walid Moallem said a Syrian warplane had attacked Khan Sheikhoun at 11:30 a.m. Tuesday — hours after the attack apparently took place and a furor had erupted on social media.
What do we know about chemical weapons?
Four years ago, hundreds of people were killed after what was said to be a sarin gas attack on rebel enclaves near Damascus. The government vehemently denied any wrongdoing, but the U.S. and others wanted Assad removed from power.
Russia stepped in with a deal that instead called for the government to surrender chemical material stockpiles to the U.N. under the supervision of Organization for the Prohibition of Chemical Weapons (OPCW).
The OPCW later said it had destroyed 95% of the Syrian government's chemical weapons, but in the years since there have been incidents where chemical materials — usually chlorine — have been deployed by the different forces in the country, including the government, the opposition and Islamic State.
What about Tuesday's attack?
Moallem said the Syrian warplane had targeted a weapons depot belonging to the former Nusra Front, now known as the Organization for the Liberation of Syria. The depot had contained chemical weapons, Moallem said.
Moallem said the Syrian army "has never used and will never use this type of weapons."
John Gilbert, a senior science fellow at the Center for Arms Control and Non-Proliferation, said the victims seen in video and photographs from Khan Sheikhoun appeared to have "been affected by something that caused serious injury, probably fatal, without any physical wounds visible … and showed symptoms consistent with the nerve agent sarin."
Sarin, if inhaled or exposed to skin in sufficient amounts, is lethal within minutes. Milder reactions include labored breathing and convulsions.
Doctors Without Borders, which supports medical clinics in rebel-held areas, said Wednesday that eight patients brought to Bab Al Hawa hospital, located near the Turkish border, had exhibited symptoms "consistent with exposure to a neurotoxic agent such as sarin gas or similar compounds."
Turkish officials said the results of autopsies on three bodies concluded that chemical weapons had been used.
Could Russia and Syria's version of events have happened?
A number of opposition activists in Khan Sheikhoun said their groups had had no rebel weapons facilities, though the town has long been a target of government warplanes.
Syrian journalist Nizar Nayouf quoted an unnamed doctor who told him there had been a storage area for the rebels in town.
"It contained a workshop … a section to store weapons and ammunition, and even foodstuffs and medical as well as logistical equipment," said Nayouf in a message on social media Wednesday.
Another activist, who asked not to be named for security reasons, said in a WhatsApp voice recording Wednesday that a bakery in the eastern part of town had been repurposed as a weapons storage area.
But Khaled Ibrahim, a pro-opposition activist, said in an interview that the bakery had been destroyed a year and a half ago in a strike and was unusable.
Why would Assad attack now?
Moallem insisted "it is not reasonable that the Syrian army could use chemical weapons now at the time when it has been achieving victories on various fronts."
Militarily, the government has achieved significant gains, while its rebel adversaries are largely in disarray. It has taken back the cities of Aleppo and Homs, and was on the cusp of negotiating issues of aid and reconstruction with world powers under the aegis of the U.N.
"Why would [Assad] do it? Because he wants to win," said Joshua Landis, director for the Center of Middle East Studies at the University of Oklahoma, in a phone interview Thursday.
"They want to win, and they thought they would try it."
Bulos is a special correspondent.