It may be the case that the Syrian military was responsible for the events last week, but recent bombings and attacks have been carried out by armed rebels with reported al-Qaeda ties. With the stakes so high, it would make sense to wait for a full investigation − unless the truth is less important than stirring up emotions in favor of a US attack.
There is ample reason to be skeptical about US government claims amplified in mainstream media reports. How many times recently have lies and exaggerations been used to push for the use of force overseas? It was not long ago that we were told Gaddafi was planning genocide for the people of Libya and the only way to stop it was a US attack. Those claims turned out to be false but by then the US and NATO had already bombed Libya, destroying its infrastructure, killing untold numbers of civilians and leaving a gang of violent thugs in charge.