Carlos Noriega -- a former lieutenant in charge of the Violent Crimes Unit for Miami Beach PD -- tells us he was the 4th person to arrive on the scene back in 1997.

He says contrary to what 'ACS' portrayed on Wednesday night's premiere episode, a tourist did NOT sneak past an officer to dip a Versace magazine ad into his fresh blood ... and then get off scot-free. Noriega says that would never happen ... and, frankly, couldn't have.

Instead of taping off a tiny perimeter around the crime scene like 'ACS' depicts, Noriega says cops taped off a much wider area around Ocean Blvd. -- and any people standing around were quite far away. He also says there weren't that many spectators on day 1.

FX had a disclaimer on the show telling viewers the series was based on true events, but that some of their depictions were "combined or imagined for dramatic and interpretive purposes."

BTW ... the whole magazine ad thing was reported in a Newsweek article back then -- an account that was never confirmed.