Oppenheimer was constantly followed by Army security. General Groves insisted that he remained on the project because he was too important to be ousted over this suspicious behavior
I have read a bit about this, and I am 100% convinced he was, at the very LEAST, a Communist sympathizer, and therefore, a security risk.
It was a quandary. He WAS the right person for the job. Flawed, but filled the piece of the puzzle.