The Big Combo (1955) on Netflix

The Police Lt. Leonard Diamond of the 93 Precinct is obsessed to find evidences against the hoodlum Mr. Brown and unsuccessfully exceeds the budget after four years of investigation with his partner, Detective Sam Hill. Police Capt. Peterson orders him to stop the investigation, telling that his obsession is personal since he has fallen in love with Brown's mistress Susan Lowell. But Susan tries to commit suicide and Diamond convinces his chief to follow one last lead, the name Alicia babbled by Susan in the hospital. The powerful Mr. Brown is mad with Diamond's investigation and sends his gangsters Joe McClure, Fante and Mingo to try to stop Diamond. But the reluctant lieutenant insists in his pursue and the criminals decide to kill him. However they murder Diamond's lover Rita by mistake. Now the obsession of Diamond becomes revenge and he will not stop until Mr. Brown is sent to justice.