How do they know what? They do know it. They know that there is no reason to suspect it at this point. What's wrong with that?
No they don't know it. They know nothing at this point. Headlines like that are premature at best since we simply do not know anything at this point.
If we really don't know one way or the other, how can we actually say "no reason...". I could just as easily say "no reason NOT to suspect terrorism -because there currently is no reason NOT to do so. It's just better to not say anything than to give the impression -and that is precisely waht that headline does- that it was NOT terrorism.