Special counsel Robert Mueller's investigators have questioned Russian oligarchs travelling in the U.S. about whether they donated to Donald Trump's 2016 presidential campaign, CNN reported Wednesday. One oligarch was questioned and his electronic devices searched when his private jet landed in the U.S., CNN said, and a second Russian oligarch was also questioned during a recent trip to the U.S. An informal interview request has been made to a third oligarch who has not recently been to the U.S., CNN said. The unusual moves suggest a ramped-up effort to find out if Russian money was illegally funneled to the Trump campaign. CNN's sources did not reveal the names of the oligarchs.