Reuters and Al Jazeera report Abdeltawwab Shahrour, a Syrian forensic medicine expert and the head of the regime's Forensic Medicine Committee, has defected to Turkey and claims to have evidence that Assad carried out a chemical attack in Khan al-Assal on March 19, five months before the alleged attack in Damascus which killed hundreds and lead President Obama to push for a military response. At the time, both the regime and the rebels accused the other of carrying out the attack that killed 25 people and injured dozens more just outside of Aleppo in the northern part of the country. 

Shahrour was scheduled to reveal his evidence of Bashar al-Assad's use of chemical weapons during a news conference in Istanbul scheduled for 10 a.m. ET, but a spokesman for the Syrian National Council appeared instead and announced the defector would not appear today, according to Al Jazeera's liveblog.

So he doesn't have evidence linking Assad to the latest chemical attack in Damascus that killed over 1,400 people and prompted threats of military strikes from the U.S., France and the U.K., but this could be the first time hard evidence links Assad to one of the many alleged chemical attacks inside Syria since the civil war began. French magazine Le Monde had two reporters embedded with the rebels through most of March and April and they witnessed the effects of many chemical attacks over that time. At one point one reporter was the victim of a chemical attack.