The BBC says it has seen evidence claiming to corroborate reports of a chemical attack in Syria in April.
A correspondent who visited the northern town of Saraqeb was told by eyewitnesses that government helicopters had dropped at least two devices containing poisonous gas.
The government has vehemently denied claims it has used chemical agents.
The United States has warned that such a development would be a "red line" for possible intervention. However, President Barack Obama has said the current intelligence on possible chemical weapon usage did not constitute sufficient proof.
On 29 April, Saraqeb, a town south-west of Aleppo, came under artillery bombardment from government positions.
Doctors at the local hospital told the BBC correspondent they had admitted eight people suffering from breathing problems. Some were vomiting and others had constricted pupils, they said. One woman, Maryam Khatib, later died.
A number of videos passed to the BBC appear to support these claims, but it is impossible to independently verify them.
Mrs Khatib's son Mohammed had rushed to the scene to help his mother and was also injured in the attack.