U.S. prosecutors are investigating an international network of traders suspected of infiltrating banks and companies to glean confidential information on megadeals, 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 U.S. 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.

The suspects appear to belong to a loose ring of more than a dozen traders spread out across London, Paris, Geneva, Dubai and other cities. Investigators suspect them of cultivating advisers, executives, lawyers and government officials with cash and gifts, the people said. These potential sources of confidential information are also being examined, they said.

U.S. prosecutors are scrutinizing the traders’ use of the media, a crucial part of their strategy, according to the people familiar with the probe. By passing on tips to financial news organizations, including Bloomberg News, the traders hoped to benefit from a pop in the stock price when reporters independently corroborated the tip and published the news, the people said.

The Justice Department probe is focusing on the traders’ conduct, including attempts to use the media for their purposes. The investigators have been reluctant to compel journalists to cooperate because of First Amendment protections, the people said.

Bloomberg News reporters talk to many people active in the market and regularly receive tips from both known and anonymous sources. The news organization’s policy is not to publish tips without confirming the information with people who have direct knowledge of the matter. The policy also prohibits telling sources when a story will be published.

Investigators believe the traders covered their tracks with the help of shell companies and complex financial instruments, the people said. They communicated on encrypted apps such as Telegram, Signal and WhatsApp with burner phones that they regularly changed or used on WiFi networks without Sim cards, the people said.

Some traders stayed at arm’s length from the operation and avoided trading on their own account, instead using associates, wealth managers and brokers to buy stock without a paper trail leading to them, the people said.

As with any investigation, the timing and outcome could change and there’s no guarantee the case will move forward, the people said.

U.S. prosecutors are sharing information with British and French investigators, the people said. They got a break when a lower-level member of the syndicate started cooperating with authorities, one of the people said. A spokeswoman for the U.K.’s Financial Conduct Authority declined to comment. A spokeswoman for France’s Parquet National Financier didn’t respond to a request for comment.

--With assistance from Gaspard Sebag, Gordana Filipovic and Bob Van Voris.