yes there is. the shooters were proven to be in contact with an Isis recruiter.
If I was in contact with an Army recruiter, shot up a bunch of anti-war protesters and the Army claimed responsibility. Would you say that my contact with a "Recruiter" would be evidence to say it was 100% them?
Being in touch with a recruiter doesn't constitute evidence. Isis could be saying it was them because it causes terror because, you know, they're TERRORISTS. They don't tend to be truthful.
However, it probably was Isis or an affiliated cell.