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.

The approach to Russian oligarchs in recent weeks may reflect that Mueller's team has already obtained records or documents that it has legal jurisdiction over and can get easily, one source said, and now it's a "wish list" to see what other information they can obtain from Russians entering the US or through their voluntary cooperation.

Foreign nationals are prohibited under campaign finance laws from donating to US political campaigns.

The sources did not share the names of the oligarchs but did describe the details of their interactions with the special counsel's team.

One area under scrutiny, sources say, is investments Russians made in companies or think tanks that have political action committees that donated to the campaign.

Another theory Mueller's office is pursuing, sources said, is whether wealthy Russians used straw donors -- Americans with citizenship -- as a vessel through which they could pump money into the campaign and inauguration fund.

The encounters with Russian oligarchs at American airports are another sign of the aggressive tactics Mueller's investigators are using to approach witnesses or people they are interested in speaking with.

"Prosecutors and investigators like the element of surprise when you can get more instinctive (and often truthful) responses," said Daniel Goldman, a former federal prosecutor, in a text. Mueller's team is using search warrants to access electronic devices and, Goldman added, "surprise is crucial for those searches because you don't want anyone to wipe their phone."

In January, FBI agents stopped and questioned George Nader, a Middle East specialist, when he arrived at Washington Dulles International Airport. They imaged his electronic devices and subpoenaed him for testimony. Nader, who attended secret meetings during the transition between the United Arab Emirates and Trump associates, is cooperating with the investigation. Nader was in the Seychelles when Trump supporter Erik Prince met with Kirill Dmitriev, the chief executive of the state-run Russian Direct Investment Fund. Prince denied any wrongdoing when he spoke with congressional investigators.

Ted Malloch, a self-described informal Trump campaign adviser, last week issued a statement saying he was stopped in Boston when returning from an international trip by FBI agents who took his cellphone and questioned him about Republican political operative Roger Stone and WikiLeaks. Malloch is scheduled to appear before Mueller's grand jury on April 13.

Late last year Mueller's team asked some witnesses if they knew of Russians who made donations directly or indirectly to the Trump campaign, sources said.

Another source added that Mueller's investigators have asked about a handful of American citizens who were born in former Soviet states and maintain ties with those countries. This person said the inquiry appeared focused on Republican fundraising and how money flows into US politics. ABC News reported in September that Mueller's team has asked questions about the timing of contributions from US citizens with ties to Russia, citing a Republican campaign aide interviewed by Mueller's team.

Trump raised $333 million for his presidential campaign, according to the Center for Responsive Politics. His inauguration committee raised a record $106.8 million, more than twice as much as any of his predecessors. Watchdog groups have criticized the committee for not fully disclosing how it spent the inauguration funds.

Another potential source of information for Mueller's investigators is Rick Gates, a former Trump campaign deputy chairman who pleaded guilty in February to financial fraud and lying to Mueller's team. Gates worked closely with Paul Manafort, who was Trump's campaign chairman for part of 2016, and stayed on as deputy chair of Trump's inaugural committee. As part of his plea agreement Gates is required to cooperate fully with Mueller's investigators and answer all their questions.

It isn't clear whether Mueller's team has identified illegal financing or if the questions are more exploratory. A spokesman for the special counsel declined to comment.

"One could say either money is fungible wherever it [ended] up," one source familiar with the inquiry said. Or Mueller's team could take the view that "you made a contribution for a purpose."