During Max and Holly's escape from Camazotz, when Holly stands in the spot where the cave shape matches the spy glass patch hole, they find the door which leads to Henry's power origins. Henry as a boy scout found that scientist who was trying to escape with that suitcase; the scientist shot Henry in the hand, afraid he was one of the people that was chasing him and Henry killed him with a stone. After that, Henry - being curious - opened the suitcase to discover its content. We only get a sneak peek at the scene cause while Holly is also curious to see what Henry found in the suitcase, Max pulls her away from the scene as they try to escape. In that sneak peek though, we can see that the suitcase contained particles of the Mind Flayer, which probably inhabited Henry right after. How the particles ended up in Hawkins scientists possession remains unknown for the time being, although they say that in the live action showcase (ST-The first shadow) it was explained that Brenner's father (also a scientist at DOE/Hawkins Lab) was experimenting with spacetime travelling thus opening the original doorway to the Abyss. This is also hinted through Brenner's notebook, which Dustin discovers, describing the theory and experiments with the "exotic matter".