Returning to Washington, Bielaski entered the Bureau of Investigation and rose to become Finch's assistant.

At the end of April 1912, Attorney General George W. Wickersham appointed Bielaski to replace Finch.

As Chief, Bielaski oversaw a steady increase in the resources and responsibilities assigned to the Bureau.

Bielaski worked undercover as a Prohibition agent operating a decoy speakeasy in New York City.

From 1929 to 1959 he headed the National Board of Fire Underwriters' team of arson investigators.