The medical and technical evidence is not consistent with a sarin attack by the Syrian government. All of the videos and pictures of the incident were taken in al-Qaeda controlled territory. All witnesses were under al-Qaeda control. How much of the incident was staged for videos (see al-Qaeda doctor video linked above) or how many of the witnesses were told to lie is not testable under current circumstance. The Syrian government insist that it has given up all its chemical weapons. The Russian government also asserts that no chemical weapon attack took place.