BY JENNY STRASBURG, MICHAEL ROTHFELD AND SUSAN PULLIAM

Federal prosecutors alleged that a "criminal club" in the hedge-fund world made tens of millions of dollars trafficking inside information, following up on a string of early morning arrests that involved employees of SAC Capital Advisors LP and other prominent financial firms.

In an expansion of a high-profile investigation into alleged insider trading, the Federal Bureau of Investigation arrested four people in New York, Boston and California, and the government unsealed charges against three others. All were charged criminally with securities fraud and conspiracy to commit securities fraud.

Court documents described enormous profits on allegedly illegal trades—including more than $50 ...