During a turbulent and contentious term in power spanning almost 50 years, J Edgar Hoover was instrumental in the fight against mounting criminality on the streets of America.
In 1924, he was appointed director of the Bureau of Investigation, which became the FBI, and he threw his weight behind the latest developments in forensic science.
His achievements were considerable but his methods were heavily criticised.
Hoover is played with scenery-chewing gusto by Leonardo DiCaprio in Clint Eastwood's handsomely crafted and slow-burning biopic.
Eastwood crafts a meticulous and elegiac portrait of the man, whose professional travails were almost as fascinating as the swirl of rumours surrounding his close relationship with FBI assistant director Clyde Tolson.
Following Hoover's death, Tolson accepted the flag draped over his mentor's coffin and inherited J Edgar's estate.
Today they are buried close to each other in the Congressional Cemetery.
Award-winning screenwriter Dustin Lance Black underpins his history lesson with a tender and chaste romance between the two.
The film opens with Hoover (DiCaprio) clinging to power, assisted as ever by loyal secretary Helen Gandy (Naomi Watts).
He begins to dictate his memoirs to Agent Smith (Ed Westwick) and drifts back in time in hazy reminiscences to the 1919 bombings which sent shockwaves through Washington DC.
With Clyde (Armie Hammer) by his side, Hoover becomes embroiled in the ill-fated search for the missing infant son of aviator Charles Lindbergh (Josh Lucas) and clashes with Robert F Kennedy (Jeffrey Donovan).
Away from the corridors of power, Hoover strives tirelessly to impress his domineering mother Anna Marie (Dame Judi Dench) who instructs her son to hold firm when others are doubing him.
J Edgar is overlong at 136 minutes and the ageing make-up used to transform DiCaprio into a liver-spotted septuagenarian isn't convincing.
However, his performance is electrifying, eyes burning bright as he tells Clyde, "Sometimes you have to bend the laws a little in order to keep your country safe."
Hammer cuts a fine figure as the loyal protege and Watts makes the most of her small role. Dench offers sterling support, sending a chill down the spine as she pointedly makes clear her views on homosexuality to her boy.
The love story, which culminates in a kiss in a hotel room and an unconventional declaration of feelings, is handled with sensitivity and restraint.