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.

Eric Schmitt contributed reporting from Washington, and Karam Shoumali from Antakya.