How do we know the man killed at the start is a cop?

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I suppose it could have been a henchman of an unmentioned rival or even a lone crook with zero context looking to rip them off, but then the questions arise of why Friedkin chose to show him displaying zero criminal characteristics, or the bad guys expressing regret at having to kill him? Seems like the most logical conclusion, supported by Occam's razor. Wouldn't bet my life on it, but probably at least a week's wages, though I'm not sure how it could be proven 100%.

Good question. They don't let the audience know he's a cop trailing Charnier. It's a flaw in the movie.

We don't