Washington (CNN) Special counsel Robert Mueller's team has taken the unusual step of questioning Russian oligarchs who traveled into the US, stopping at least one and searching his electronic devices when his private jet landed at a New York area airport, according to multiple sources familiar with the inquiry.

A second Russian oligarch was stopped during a recent trip to the US, although it is not clear if he was searched, according to a person briefed on the matter.

Mueller's team has also made an informal voluntary document and interview request to a third Russian oligarch who has not traveled to the US recently.

The situations have one thing in common: Investigators are asking whether wealthy Russians illegally funneled cash donations directly or indirectly into Donald Trump's presidential campaign and inauguration.

Investigators' interest in Russian oligarchs has not been previously reported. It reveals that Mueller's team has intensified its focus into the potential flow of money from Russia into the US election as part of its wide-ranging investigation into whether the Trump team colluded with Russia's interference in the 2016 presidential election.