That's probably what the bomb looked like. Reports say that the fake bomb was made from batteries, a watch, an empty gas cartridge and with a photograph of Hulk taped to it. Recently two players were sent to train with the reserves after they publicly voiced their disgust at the wages of Hulk and Alex Witsel. I would suggest the police start there.
A message attached to the bomb simply read "Hulk Out!". Not the most creative threat but it sure does get the point across. He could be referring to the dance craze that has hit Russia called "the Hulk". Someone shouts "Hulk out!", you have to change into a green morph suit and pretend you are having a rage induced seizure. Try it next time you are out and watch the ladies swoon.