How would you explain “on to” in the context? “Anyway, we proved beyond doubt Mrs Bestigui couldn’t have heard it. The Bestiguis moved into a friend’s house the next day to get away from the press, so we put a few blokes in their flat, and a guy up on Landry’s balcony, shouting his head off. The lot on the first floor couldn’t hear a word he was saying, and they were stone-cold sober, and making an effort. “But while we were proving she was talking shit, Mrs. Bestigui was phoning half of London to tell them she was the sole witness to the murder of Lula Landry. The press were already on to it, because some of the neighbors had heard her screaming about an intruder. Papers had tried and convicted Evan Duffield before we even got back to Mrs. Bestigui. How would you explain “on to” in “The press were already on to it”? Thanks!