[JoeFromSidney]

How did the cop know Gray was carrying a knife before he was arrested? If Gray had the knife in his pocket, it was invisible, and could not have created probable cause. If the knife was found only after Gray was stopped for something else, it might then be a crime if Gray was under disability.

[FredZarguna]

His running away constitutes reasonable suspicion but not probable cause. On reasonable suspicion they can question him and perform a superficial pat down. So they could have discovered the knife well within the parameters of what's allowed.

See, for example:

Running From the Police