Special Counsel Robert Mueller's team has pulled over at least one Russian oligarch, arriving in New York by private jet, to search the individual's electronic devices, CNN has learned.

The network is also reporting that Mueller's team stopped a second Russian, and reached out to a third making an informal and voluntary document and interview request.

Investigators want to know whether wealthy Russians funneled cash – directly or indirectly – into the Trump campaign or the president's inauguration fund, CNN said.

Special Counsel Robert Mueller's team stopped a Russian oligarch as he arrived in New York City on a private jet and searched the individual's electronic devices, CNN reported Wednesday

Special Counsel Robert Mueller's (pictured) investigators are asking Russian oligarchs if they directory or indirectly financed President Trump's campaign or his inauguration, the network learned

Foreign nationals are barred from contributing money to American political campaigns.

None of the three Russians were identified by CNN's sources.

The network's reporting has revealed a new string being pulled in the ever growing Mueller probe, with investigators' interest in oligarchs previously unknown.

It also shows that Mueller's team hasn't shied away from using aggressive tactics.

'Prosecutors and investigators like the element of surprise when you can get more instinctive (and often truthful) responses,' former federal prosecutor Daniel Goldman explained to CNN in a text.

Because Mueller's team is obtaining search warrants to access electronic devices, 'surprise is crucial for those searches because you don't want anyone to wipe their phone,' Goldman added.

CNN learned that investigators are interested in links between Russians and think tanks or companies that have political action committees that may have donated to the president's campaign.

Mueller's people also want to know if Russians may have used so-called 'straw donors' – American citizens – as a way to pass off cash to the campaign or the inaugural fund.

It was unclear whether the special counsel's team has unearthed evidence of illegal financing or if the questions were simply exploratory in nature.

A spokesman for the special counsel didn't comment to CNN.