1934
Paul Blanshard is appointed DOI Commissioner by Mayor Fiorello H. LaGuardia who said he would be "his eyes and ears." DOI Complaint Bureau established. Approximately 6,000 complaints received during the first year. DOI becomes known as the City's "Watch Dog." During the first three years of LaGuardia's administration, some 94 officials are ousted or resign because of investigations.

1938
DOI assigned formal responsibility for supervising and regulating the City Marshals.

DOI's agency name formally changed to the Department of Investigation.

1946
John Murtagh appointed DOI Commissioner. He considers job at DOI to act "as the bromo-seltzer for the Mayor."

1954
The City Bureau of Marshals established in DOI.

1961
The City establishes the Post Office "Box 100" anti-corruption program, which enables citizens to write directly to the post office box to report "graft or unethical practices."

1970
Establishment of Executive Order No. 21, which, in part, requires applicants for all management positions and persons appointed to a position of a sensitive nature, "to complete a DOI personal history questionnaire and related investigation, prior to beginning employment." The Order also established a uniform method for each mayoral agency to handle allegations concerning "competency and integrity" of municipal employees, calling for one or more individuals to be responsible at the agencies for receiving these complaints and making them obligated to notify DOI about serious allegations.

1977
Section 807 of the 1975 revised City Charter takes effect, requiring that the Commissioner of DOI approve the appointment of each Inspector General by the agency head and "promulgate standards of conduct and . . . monitor and evaluate the activity of Inspectors General in the agencies to assure uniformity of activity by them."

After a two-year demonstration period, the Corruption Prevention and Management Review Bureau is established within DOI. The objective of the Bureau is to identify systemic and management failures and correcting these, concentrating on corruption prevention.