(4.5.2018) Federal agents working with Mr. Mueller stopped Mr. Vekselberg, a billionaire businessman, at a New York-area airport this year, searched his electronic devices and questioned him, according to people familiar with the matter. They confronted him after he stepped off a private plane about two months ago, according to one of the people.

There is no indication that Mr. Mueller suspects Mr. Vekselberg of wrongdoing. But Mr. Vekselberg attended the presidential inauguration last year,