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.

Investigators also have evidence that the late GOP activist Peter W. Smith may have had advance knowledge of details about the release of emails from a top Hillary Clinton campaign official by WikiLeaks, one person familiar with the matter said. They have questioned Mr. Smith’s associates, the person said.

Right-wing pundit Jerome Corsi was also questioned by investigators about his interactions with Mr. Stone and WikiLeaks before a grand jury in September, according to a person familiar with the matter. Mr. Corsi declined to comment. A lawyer for Mr. Stone said he hasn’t been contacted by the special counsel. Mr. Smith died last year.