U.S. prosecutors are investigating an international network of traders suspected of infiltrating banks and companies to glean confidential information on mega-deals, according to people familiar with the matter.

The probe by prosecutors at the U.S. Attorney's Office for the Southern District of New York is focusing on a group of stock pickers in Europe and the Middle East who have made tens of millions of dollars trading ahead of media reports about takeover talks or merger announcements by companies, according to the people, who asked not to be identified because the matter isn't public. The investigation is part of a years-long multinational effort, with U.S., U.K., and French prosecutors operating on parallel tracks.

The first hint of U.S. involvement came in November, when authorities in Serbia arrested one of the suspects, a Geneva-based trader, on a U.S. warrant that alleged he had committed securities fraud. He was extradited to the United States in May, according to Tatjana Pesic, a court spokeswoman in Belgrade. Dawn Dearden, a spokeswoman for Manhattan U.S. Attorney Geoffrey Berman, declined to comment.