Given the question: Build a movie plot around this: hat was the F.B.I. investigating? Narcotics smuggling
The answer is:
Roy Roberts, Claire Trevor and Fred MacMurray in BorderlinePete Ritchie (Raymond Burr) runs a narcotics smuggling operation to the USA from Mexico, which the Los Angeles Police Department and the US federal government have unsuccessfully tried to stop. Because of Ritchie's careful operating procedures, US authorities haven't even been able to find out the identities of his sources or customers and are desperate for a breakthrough. As a last resort, Madeleine Haley (Claire Trevor), an LAPD officer and former OSS operative, is sent undercover to Mexico to charm her way into Ritchie's confidence.Once there, Haley manages to establish contact with Ritchie's gang, but is kidnapped by Johnny Macklin (Fred MacMurray), a federal agent posing as a hoodlum working for a rival of Ritchie's and who also steals a load of Ritchie's narcotics. Haley plays along and joins Macklin on a smuggling trip for the stolen narcotics, with Ritchie in hot pursuit.