In those days Hollywood often used an artificial so-called mid-Atlantic accent, no doubt to pander to as wide an audience as possible. I'd be a bit surprised if Cagney was not using a real US accent, at least as a gangster, but perhaps writers, too, tried to straddle the Pond? We see a variation of that now in British-made films and tv programmes where writers try to insert Americanisms like 'gotten' to widen their markets. I object, not because it's American but because it sounds so artificial. I'd like to think Americans would object, too, on the additional ground that it is being assumed they are ignorant.Copper was certainly used in American films before the second world war. In particular James Cagney used the term:
I see the OED says copper is "Brit. informal", which I had assumed anyway. We use the verb cop (=catch) quite widely and I'm not aware that Americans do.
More on the "route" and "en route" - I got yet another variation on Chicago PP last night: "enn rowt" as opposed to the previous "on rowt". Why don't they just say "I'm on my way?" It's like "I've got eyes on him" instead of "I can see him". Along with "copy that" and "affirmative", is this all supposed to convey a tough-guy military flavour among cops? If so, it seems to me they could do with a change of PR consultants in the light of recent events.