In your minds, racism is the only reason that a person could possibly think radical Muslims were behind the attack. In reality, there is a lot of statistical and circumstantial factors that lead people to suspect this.
Even from a psychological perspective, people are the most fearful of radical Muslims based on what they've done in recent history. They are afraid that they will become more aggressive in their attacks. "Oh what an asshole. He's fearful that it was those people that have committed attacks in the recent past and explicitly state that they want to kill all Americans are becoming more aggressive".
Of course, the understanding that would result from a psychological perspective such as this doesn't count unless it supports the PC liberal cause.
Any opinion on this specific subject that doesn't put a tremendous amount of weight on what radical Muslims have done in recent history and what radical Muslim groups have explicitly stated regarding committing terror attacks is a misguided opinion. Come to think of it, it really isn't much more complicated than that.