Special counsel Robert Mueller’s investigators are examining comments by Roger Stone—a longtime adviser to Donald Trump—about WikiLeaks during a series of conference calls he hosted in 2016, according to a witness in the probe and another person familiar with the matter.

In at least two August conference calls advertised online to the public and promoting himself as “the ultimate political insider,” Mr. Stone told callers about WikiLeaks’ plans to release information that he said would affect the 2016 presidential campaign...