He spent many years combating counterfeiting, which led to his investigation and arrests of Black Hand extortionists and members of the American mafia, many of them associated with the Morello crime family.

Petrosino's murder was never officially solved, but the author and historian Mike Dash implicates the likely gunman and his accomplice and says there is little doubt that Giuseppe Morello was behind it.

[2] Flynn and his operatives built the case that culminated in the 1910 convictions of Morello and his associates and their imprisonment in Atlanta Federal Prison.

In 1915 he investigated espionage involving a German-owned wireless station on the coast of Long Island in Sayville, New York.

Flynn took charge of hunting down the bombers and assigned "an ambitious Justice Department clerk by the name of J. Edgar Hoover" to monitor suspected radicals.

[6] Flynn also became a scenario writer for the motion picture industry through his acquaintance with the actor King Baggot who, Dash notes, was considered the greatest film star in the country at that time in 1912.

The producers Theodore and Leopold Wharton commissioned him to write story lines for their films, including The Perils of Pauline, and eventually adapted Flynn's experiences into a 20-part spy thriller titled The Eagle's Eye (1918), starring Baggot.

[5] The same year they were also published as weekly installments in The Atlanta Constitution's magazine section under the title The Eagle's Eye: True Story of the Imperial German Government's Spies and Intrigues in America.

[8] He also edited a magazine which bore his name, Flynn's Weekly Detective Fiction, and became the longest-running, most successful journal of its genre.