The population of France is about 7.5% Muslim. France has had a close relationship with the Muslim world since the 19th century when they colonized parts of North Africa and the Middle East. Sadly, the position in French society of colonial immigrants and their descendants is not very high. Also sadly, France has had a lot of experience with this sort of violence, both in the colonies and "at home".
If you know the attacks were carried out by Muslims, there's no reason to assume these attacks were carried out by refugees.