So far, officials say, two factors make them wonder whether the facilities were still central to Syria’s program. At this point, there are no known casualties at the sites, which suggests that either no one was there during the evening, or they had been previously abandoned. And there are no reports of chemical agent leakage from the sites, despite attacks by more than 100 sea- and air-launched missiles.
Whether those particular sites were still in use or not, the conflict in Syria has demonstrated a larger truth: While it is easy to blow up Mr. Assad’s chemical facilities, it is also relatively simple for him to reconstitute them elsewhere, or just turn to a commercially available substance like chlorine to make a crude poison that any nation is allowed to possess.
That may explain why General McKenzie was a little more circumspect than his predecessors in forecasting the long-range effectiveness of the latest strikes.
“I would say there’s still a residual element of the Syrian program that’s out there,” he told reporters at the Pentagon on Saturday.
“I believe that we took the heart of it out with the attacks that we accomplished last night,” he continued. But he added that “I’m not going to say that they are going to be unable to continue to conduct a chemical attack in the future.” Instead, he said, he believed that “they’ll think long and hard about it.”
His comment seemed to reflect a central fact: Mr. Assad has learned a lot about how to hide his stockpiles from inspectors. One of the failings of the accord between Russia and the United States that was supposed to rid Syria of chemical arms in 2014 was that it was based on Syria’s “declared” stockpiles, a nuance Mr. Kerry took care to note. There was never confidence that the Russians had succeeded in removing as many weapons stores as they claimed, or in destroying production facilities.
One of the sites struck on Saturday, the Barzeh research and development center, has long been known to American intelligence officials and to international inspectors, according to a senior intelligence official.
Administration officials briefing reporters Saturday afternoon said that equipment at that site and two others — the Him Shinshar chemical weapons storage facility and a nearby “bunker facility” — was destroyed, setting back Mr. Assad’s program by months or years. But they were careful not to claim that the facilities were actively in use at the time of the attack.
“In relation to the Barzeh target, yes, we assessed that there were probably some chemical and nerve agents in that target,” General McKenzie said. Those, however, could have been residual, or long abandoned.
Sophisticated facilities are not needed to produce chlorine, the agent that Syrian forces are suspected to have used a week ago to kill dozens of civilians, including children. It is commercially available, used for water systems. And the nerve agent sarin, which the White House has said may also have been used a week ago, can be produced just about anywhere, as a French intelligence report released on Saturday noted.
The report concludes that “the Syrian military retains expertise from its traditional chemical weapons agent program to both use sarin and produce and deploy chlorine munitions.” The United States, it noted, “also assesses the regime still has chemicals — specifically sarin and chlorine — that it can use in future attacks.”
That is a stark difference from the declarations made in June 2014, when the joint mission between the United Nations and the Organization for the Prohibition of Chemical Weapons trumpeted that Syria or other parties had destroyed, with the exception of a dozen facilities, all “declared production, mixing and filling equipment and munitions, as well as many buildings associated with its declared chemical weapons program.”
Chlorine presents a particular challenge for governments and organizations seeking to control chemical weapons. It is both very lethal and very necessary, and legal to possess in all nations, said Michael Knights, a fellow at the Washington Institute for Near East Policy.
Mr. Knights, who studied the use of chlorine as a weapon by Al Qaeda in Iraq a decade ago, said that the militants used to pair it with explosives, which would burn it off. Using it the way Mr. Assad’s forces have — essentially dropping it in concentrated form in a barrel — can lead to far more casualties.
“The most dangerous prospect was that someone would use chlorine in the way that the Syrian regime has done and disperse it in a whole community,” he said. “It is a dangerous element, and you cannot remove it from any environment in the Middle East,” given its importance in water purification.
As a result, chemical weapons treaties do not bar its possession. But the Chemical Weapons Convention, which went into force in 1997 and which Syria joined, under pressure, in 2013, does bar deliberately using it as a poison gas.
Nations have been finding loopholes in such international agreements — or ignoring them entirely — since the first failed efforts to bar such weaponry more than a century ago. Mr. Assad has clearly calculated that turning a widely available chemical into a weapon to clear out neighborhoods and create terror is a potent option.
“For me, the big story is chlorine; it’s not sarin,” Mr. Knights said. “The regime has been good at using a chemical weapon that has enormous availability and is produced in completely legal, dual-use facilities and vital to the running of any country.”
“Intent is the problem when you’ve got a regime that loves using this stuff,” he added. “You either have to deter the regime from using it by imposing significant costs, or you have to get rid of the regime. But there is no way you can get rid of the capability.”
No international investigative body has yet determined what chemicals were used in the attack last Saturday in Douma, near Damascus. The American intelligence assessments suggest that while chlorine was the primary chemical, “some additional information points to the regime also using” sarin, which is more dangerous and harder to handle.
Videos taken in the aftermath of the attack show large yellow canisters that experts have said appear to be chlorine tanks, of the kind often used for civilian purposes.
Some videos show one canister that appears to have either broken through a wall or flown through a hole in it and landed on a bed without exploding.
Another video shows a similar canister that appears to have knocked a hole in the concrete roof of an apartment building.
Syrian activists who visited the site, as well as a report by Bellingcat, a group that conducts investigations with open-source data, said that the canister had fallen on the roof of a building where dozens of people had been sheltering on the lower floors.
The available evidence suggests that whatever substance was in the canister was released after it landed and wafted down the stairs, killing those on the floors below.
Subsequent videos show dozens of men, women and children lying lifeless on the ground with foam coming from their mouths and nostrils and what appear to be burned corneas. Both symptoms can be caused by chlorine.
In a series of tweets after studying the attack, Eliot Higgins, the founder of Bellingcat, said that the canister appeared to have been dropped toward no specific target, and had just happened to land on a densely packed building.
Had the canister fallen elsewhere, Mr. Higgins wrote, “we’d see a fraction of deaths, and you probably wouldn’t have even heard about it happening.”
“It was really just thanks to the catastrophic success of the chlorine attack that anyone even cared, not anything that Assad could have planned for,” Mr. Higgins wrote.