Just after midnight on April 25th, a Syrian medical technician who calls himself Majid Daraya was sitting at home, in the city of Daraya, five miles from the outskirts of Damascus, when he heard an explosion. He ran outside, and, on the southern horizon, he saw a blue haze. “I’ve never seen a blue explosion before,” he remembers thinking. Seconds later came another blast, and another blue haze. Majid, who used a pseudonym to protect his identity, told me that his city had become a violent and unpredictable place; for five months, it had been the scene of heavy combat between forces loyal to the regime of Bashar al-Assad and the rebels who have been fighting for more than two years to drive him from power.

Within a few minutes, Majid said, his eyes began to burn, and he felt sick to his stomach. He decided to walk to the local hospital, where, as an anesthesia specialist, he spent most of his daytime hours. When he arrived, dozens of people were streaming in, choking, vomiting, crying, saliva bubbling out of their mouths. About a hundred and thirty people were treated for similar symptoms; ten of them, Majid said, were in “dangerous” condition, though none died. The victims were suffering from chemical poisoning, but there wasn’t much that the doctors could do except try to alleviate the symptoms. “We don’t have medicine to cure that kind of poisoning,” Majid said, in a telephone interview. (We had been introduced by the Syrian Support Group, a pro-opposition organization in Washington, D.C.) “The people were terrified, because no one could help them.”

On the way home, Majid saw birds and other animals—goats, chickens, stray dogs—writhing on the ground. Others were dead. “All these birds and chickens were dead around us,” he told me. “I can’t describe the fear that people felt.” A statement by the rebel-led city council said that the regime had used sarin and possibly chlorine gas. The council members held the Syrian government responsible and called on the international community to “find out the truth about the killing machine.” Majid directed me to a macabre gallery of photographs and videos, posted online by opposition leaders in Daraya. “It was poison gas,’’ he said. “It affected the birds and the animals and the humans in the same terrible way.”

Since March, there have been reports of at least four similar attacks, including one in Ateibeh, a contested area near Damascus, and one in Khan al-Assal, a town outside Aleppo. The reports indicated that the attack in Khan al-Assal had killed twenty-two people and injured forty-eight, and that the one in Ateibeh had contaminated as many as twenty-five people. Majid’s account could not be independently confirmed. An American intelligence official told me that he had learned of the purported attack, and others, by monitoring rebel Web sites. Like the other attacks, the one in Daraya was shrouded in ambiguity. What was the gas that Majid described? Was it a substance banned by international treaty, like sarin or VX? Or was it something less virulent? Had the attack been ordered by Assad, or had it been carried out by a Syrian military unit operating on its own authority? (Although the regime has accused rebels of such attacks, American officials believe that they don’t have chemical weapons.) And, if the incidents reported by Majid and other Syrians did amount to a use of chemical weapons, what could be done to prevent the next one?

On several occasions, President Obama has declared that if the regime used chemical weapons, or even prepared to use them, it would be crossing a “red line.’’ But the Administration has taken care not to make the line too sharp, referring not just to chemical weapons but to “a whole bunch” of chemical weapons, used in a “systematic” way. And though Obama has said that such attacks would be a “game changer,” he has stopped short of saying that they would be cause for military force.

Joseph Holliday, a former Army intelligence officer who has studied the conflict for the Institute for the Study of War, in Washington, suggested that the regime was attempting to use the weapons in a way that would frighten the rebels but wouldn’t cross the red line. “Assad has been extremely calculating with the use of force, increasing the levels of violence gradually, so as not to set off alarm bells,” he said. “First it was artillery. Then it was bombing. Then it was Scuds. A year ago, he wasn’t killing a hundred people a day. He’s introducing chemical weapons gradually, so we get used to them.” The attacks in March and April took place in areas that were either contested or held by the regime, and they killed relatively few people, at a time when, elsewhere in the country, a hundred people were dying every day. “If it’s not a big attack, it’s not easy to determine whether chemical weapons have been used,” a Senate aide told me. “The cloud disperses—there’s no mushroom cloud. Maybe Assad bombards the area afterward to cover up the evidence.” Indeed, some experts said that the regime was using the attacks specifically to gauge the resolve of Obama and the West. “Assad appears to be testing the tactical value of his chemical arsenal,” Gary Samore, who until February was President Obama’s chief adviser on weapons of mass destruction, said. “But he’s testing the political limits, too.”

Senior Israeli officials and Republicans in Washington, as well as British and French intelligence officials, have argued forcefully that the regime used chemical weapons. The Administration’s response has been characterized by caution, indecision, and reluctance to speak publicly about the subject. Officials said in late April that they believed chemical weapons had been used at least twice, but that they could not definitely tie the attacks to Assad. The White House said that it was not entirely clear who was in control of the weapons, leaving open the possibility that the attacks were accidental or unauthorized. “Given the stakes involved, and what we have learned from our recent experience, intelligence assessments alone are not sufficient,” the White House wrote in a letter to congressional leaders. Instead, the Administration would rely on the United Nations, which planned to send in experts to test soil and take samples from victims. Assad refused to allow the experts into the country.

A White House aide told me, “There is no question in our minds that the regime would be willing to use these weapons, is able to use these weapons, and is increasingly likely to use these weapons as things continue to go badly for them.” But, at a recent meeting at the State Department, according to a person who attended, “No one wanted to say that Assad had crossed the line, because no one wants to deal with it.” Assad’s chemical arsenal is spread across the country, much of it in populated areas; an effective military strike against it would need to be huge, and meticulously coördinated, to make sure that no toxins were released into the air or into enemy control. Samore told me, “It’s really a nightmare military scenario.’’ As the regime has traded ground with the rebels, some of Assad’s chemical weapons have been moved, and it is not clear where all of them are. “The intelligence people told us that their visibility is basically zero on some of these weapons, that we’re not going to know until after they have been used—if then,’’ the Senate aide told me.