Previous to that service, he led a far-reaching investigation into corruption in the New York City Police Department from 1970 to 1972.

[4] In 1941, Knapp returned to private life and joined the law firm of Donovan, Leisure, Newton & Lumbard.

In 1950, Knapp left Mr. Hogan's office to again enter private practice until his appointment to the federal bench in 1972.

[citation needed] The probe was sparked by revelations from two police officers, Patrolman Frank Serpico, and Sergeant David Durk.

[citation needed] In 1993, Knapp joined with Judge Jack B. Weinstein of the United States District Court for the Eastern District of New York, based in Brooklyn, New York, in declaring that they would no longer preside over drug trials.