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...