Federal authorities are seeking to build insider-trading cases against roughly 120 individuals on and off Wall Street in an expanding criminal insider-trading investigation that has shaken the financial and corporate worlds.

The disclosure—the first time authorities have quantified the number of people under scrutiny—comes on the heels of a string of successful prosecutions of insider trading. Since late 2009, prosecutors have charged 66 individuals at hedge funds and other companies with insider trading and won 57 convictions...