A new study of images apparently from the Syrian attack last month concludes that the rockets delivering toxic sarin gas to neighborhoods around Damascus held up to 50 times more nerve agent than previously estimated, a conclusion that could solve the mystery of why there were so many more victims than in previous chemical attacks.
The study, by leading weapons experts, also strongly suggests that the mass of toxic material could have come only from a large stockpile. American, British and French officials have charged that only the Syrian government and not the rebels was in position to make such large quantities of deadly toxins.
Secretary of State John Kerry told Congress, in hearings on Tuesday and Wednesday, that the United States believes the Syrian military was responsible for the attack, and in classified briefings officials have pointed to Unit 450, which controls Syrian chemical weapons.
The new study was conducted by Richard M. Lloyd, an expert in warhead design, and Theodore A. Postol, a physicist at the Massachusetts Institute of Technology. They based their investigation on scores of online videos and photographs posted since the Aug. 21 attack sent thousands of sick and dying Syrians to hospitals in the Damascus suburbs.
In interviews and reports, the two weapons specialists said their analysis of rocket parts and wreckage posted online suggested that the warheads carried toxic payloads of about 50 liters (13 gallons), not the one or two liters (up to half a gallon) of nerve agent that some weapons experts had previously estimated.
“It’s a clever design,” Dr. Postol said of the munitions in an interview. “It’s clever not only in how it was implemented but in the effectiveness of its dispersal. It accounts for the large number of causalities.”
Shortly after the attack, some analysts said they doubted if the identified rockets could have carried enough nerve agent to have caused the mass casualties. Mr. Lloyd and Dr. Postol say their analysis explains how the misidentification of a central rocket part resulted in the excessively small payload estimates.
In an interview, Mr. Lloyd said the manufacture of the rockets, if not the deadly nerve agent, appeared to be within the capabilities of both the Syrian government and the rebels.
But Stephen Johnson, a former British Army chemical warfare expert who is now a forensic expert at Cranfield University, at Shrivenham, said if the estimate of a 50-liter payload was correct, only the Syrian government could have achieved such a large volume of production.
“That’s a fairly substantial amount to produce yourself and beyond the opposition in its wildest dreams,” he said. Suggestions that the Syrian rebels seized or secretly obtained such amounts, Mr. Johnson added, lacked credibility. “It’s more supportive of the argument that it was the government,” he said.
The Obama administration has charged that the Syrian government fired rockets carrying warheads filled with sarin, a liquid nerve agent that vaporizes into a deadly mist that human skin can quickly absorb. The toxin throws nerves and muscles all over the body into overdrive, resulting in lung paralysis and death. The pupils of victims are often tiny because the iris, a muscle, contracts so much.
In their analysis, Mr. Lloyd and Dr. Postol said experts analyzing pictures of the rocket debris in Syria had misidentified thin tubes found sticking out of the ground as the payload canister. Instead, they say, the tubes made up an inner explosive device that, when the rocket slammed into the ground, caused a much larger container to burst open and disperse large volumes of gas.
Photographs of impaled rockets, the weapons experts say, often show the crumpled skin of the larger canister lying nearby.
“This design explains the evidence on the ground,” Dr. Postol said. The cloud from the impacting rocket, he added, probably rose to a height of 10 or 15 feet.
Dr. Postol is a professor and national security expert in M.I.T.’s Program in Science, Technology and Society. Mr. Lloyd, in two decades at Raytheon, a top military contractor, wrote two books on warhead design and now works for Tesla Laboratories, a military contractor in Arlington, Va.
Raymond A. Zilinskas, a senior scientist at the Monterey Institute of International Studies and a former United Nations weapons inspector, said the analysis of the two weapons experts seemed plausible. He said that deadly rockets that Iraq fired at Iran in the 1980 held nine liters of toxic chemicals, and that the Syrian rockets involved in the massacre looked like those but with an added secondary canister.
“I can’t say if it was 50 liters,” Dr. Zilinskas said, “but it would certainly add to the payload.”
David E. Sanger contributed reporting.