As for your example, the word "дело" there means "case". Like in a phrase "a criminal case".
In some cases we can also use a phrase "по обвинению в покушении" instead of "по делу о покушении". "По обвинению в покушении" = "on charges of assassination attempt".
Quote Originally Posted by George1992 View Post

Does this "по" mean "by (means of)"? Something like -> Cut the bread with this knife.
I don't think so.