ANTAKYA, Turkey -- A Syrian doctor said that one day in March, in the city of Aleppo, he was on duty in a hospital when it suddenly filled up with patients, some vomiting, choking or sensitive to light. A few responded to a treatment for nerve gas, he said, but at least two dozen others died.
The doctor, who spoke on the condition that he not be identified for his own safety, said he and the other doctors believed the patients were exposed to a nerve agent or pesticide, but "we were not able to determine what the inhaled substance was."
Nearly three months later, this episode remains unexplained -- and is one of several events that appear to add weight to claims that chemical weapons have been used in Syria. But if so, why has it been so hard to say for certain?
The United States and Britain have said that they have detected sarin, a nerve agent, in physiological samples from Syria and that it was probably used by government troops. French officials have gone further, saying there is "no doubt" that the Syrian government used sarin in at least one attack and possibly others.
But none of the evidence has been made public, and many experts on chemical weapons say that it is important to remain skeptical, that the anecdotal evidence that has emerged is inconclusive and needs to be investigated by an impartial organization. Some experts have been mystified by the relatively low number of deaths, given the toxicity of a nerve agent like sarin. They are also confused by the range of symptoms seen in videos disseminated by Syrian opposition activists -- including some that seem mild -- leading to questions about what kind of toxins were used, but also the veracity of some of the videos.
"There is probably something out there," said Jean-Pascal Zanders, a chemical weapons expert who has closely followed the events in Syria. "But I don't know what it is."
Adding to the uncertainty, some experts said, is the incentive that President Obama may have unintentionally provided to exaggerate the reports. Last August the president said that the use of chemical weapons by the Syrian government would cross a "red line" and "change my calculus" on whether the United States should intervene in Syria -- which is exactly what many of Mr. Assad's opponents have hoped for.
"There's a rush to draw conclusions that a red line has been crossed," said Joost Hiltermann, chief operating officer of the International Crisis Group, who wrote a book analyzing the use of chemical weapons during the Iran-Iraq war. An investigation into the current allegations, he said, should rely on experts not aligned with states that have a stake in the war, to rigorously establish the source of soil, physiological or other samples.
"We don't know anything yet," Mr. Hiltermann said. "Let's be very careful."
Accusations of chemical weapons use intensified after a journalist from the French newspaper Le Monde, Laurent van der Stockt, said he was sickened last month by what he believed to be sarin while he and a colleague were traveling with rebel fighters in the Damascus suburbs. Alistair W.M. Hay, an expert on the effects of chemical weapons at the University of Leeds in England, said the string of symptoms the reporters described were "convincing" indications of exposure to a nerve gas, rather than a riot agent like tear gas.
The reporters took urine samples from victims of an attack in the suburb of Jobar, and delivered them to the French government.
The French foreign minister, Laurent Fabius, said in a statement last week that those samples had tested positive for sarin but that investigators could not establish who had used it. He also said that a batch of blood samples taken from the victims of a government helicopter attack in the northern Syrian town of Sarqib also tested positive for sarin, leaving "no doubt" that it was the government that used the gas.
United Nations investigators in Geneva also reported on the same day that they had found "reasonable grounds to believe limited quantities of toxic chemicals were used" in Aleppo.
But while such evidence has not been made public, it may well be the events in Aleppo in March that best illustrate why the picture remains murky.
That attack on March 19 in Khan al-Assal, a village that was seen as sympathetic to the government and at the time was under the military's control, drew attention because of the death toll, and because both the government and its opponents accused each other of using chemical weapons in the attack.
That raised a number of confusing possibilities: that the government was trying to frame its opponents; that it had accidentally attacked its own troops, who were among the victims; or that insurgent groups possessed chemical weapons and were willing to use them.
After hearing about the attack, a group of doctors associated with the Syrian American Medical Society tried to smuggle blood samples out of the country to be tested. Dr. Yahia Abdul-Rahim, a pediatrician originally from Aleppo who now lives in the United States, contacted someone he knew at the University Hospital in Aleppo, where many of the victims were taken, and asked the person to draw blood samples from those who died. Dr. Abdul-Rahim said he then reached out to a contact in the United States government, telling them about the samples.
Dr. Abdul-Rahim refused to supply the name of the Aleppo hospital worker or of his government contact. His account could also not be confirmed.
Dr. Abdul-Rahim said a civilian courier took the samples from Aleppo to the Turkish town of Reyhanli, a journey that took longer than expected. At one point, the courier forgot the blood vials, which were not refrigerated, in his car. Ten days after the attack, the vials arrived at the Turkish field office for the Syrian American Medical Society.
In late April, Dr. Abdul-Rahim said, American officials requested a meeting to ask questions about the blood samples from Khan al-Assal attack, as well as another batch from a separate attack. During the two hour meeting, he said, the officials did not reveal what, if anything, they had discovered.
"They wanted to make sure there was no tampering with the samples," Dr. Abdul-Rahim said. They also asked for details about the victims.
Within a day or two -- Dr. Abdul-Rahim is not sure -- the Obama administration delivered its strongest confirmation that Syria had used chemical weapons. In a letter to Congress, the White House said that American intelligence agencies had concluded with "some degree of varying confidence" that the Syrian government had used sarin. Several officials said the agencies had expressed medium to high confidence about their assessment.
Soon after the statement, Dr. Abdul-Rahim received a call from his government contact "congratulating me."
"It is because of the samples? I don't know," Dr. Abdul-Rahim said. "He gave me the impression the meeting was fruitful. Whether he was just telling me that to make me happy, I don't know. "
The State Department declined to comment on whether it had met with the Syrian-American doctors, or to say what part if any their samples played in determining the use of sarin.
An American official who was granted anonymity because he was not authorized to talk publicly said that investigators had found sarin in physiological samples, including some from victims in Khan al-Assal, although he did not say whether it was the doctors who had supplied them.
But even then, the samples raise as many questions as they might answer. "What it cannot tell us is when it was used, where it was used, who used it and what were the circumstances," the official said. "The physiological examples are compelling but without being able to determine the chain of custody, that's the key to confirming the use."
That uncertainty has been little comfort to Syrians terrified of such attacks, or doctors determining how to prepare for them. At the same time, the issue strikes many as a distraction from the brutality of Syria's war, where tens of thousands have been killed by conventional weapons.
Dr. Abdul-Rahim said his primary interest was protecting people inside Syria.
"I'm not trying to test President Obama," he said. "I'm trying to prove to the international community how much silence there is."
Eric Schmitt contributed reporting from Washington, and Karam Shoumali from Antakya.
This article originally appeared in The New York Times.