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.