WASHINGTON—Special counsel Robert Mueller’s investigation is scrutinizing how a collection of activists and pundits intersected with WikiLeaks, the website that U.S. officials say was the primary conduit for publishing materials stolen by Russia, according to people familiar with the matter.

Mr. Mueller’s team has recently questioned witnesses about the activities of longtime Trump confidante Roger Stone, including his contacts with WikiLeaks, and has obtained telephone records, according to the people familiar with the matter.

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