They have it narrowed down to a few thousand feet. To call it "unknown" is just unfair. They might not know which steel plant is producing it, but they can narrow it down to a small handful, far from "unknown". They have enough evidence of which company is producing the sound (United States Steel), so the only thing stopping them from getting police or lawmakers involved is the fact that no one on the US side cares enough. The article could have mentioned this caveat, but after the couple sentences about the likely cause, it's followed by more "It’s like chasing a ghost" rhetoric.