While outwardly quiet for the last month, Robert S. Mueller III’s investigators have been aggressively pursuing leads behind the scenes about whether Stone was in communication with the online group, whose disclosures of emails believed to have been hacked by Russian operatives disrupted the 2016 presidential campaign, according to people familiar with the special counsel probe.

Stone, who boasted during the race that he was in touch with WikiLeaks founder Julian Assange, has said since that his past comments were exaggerated or misunderstood. Both he and WikiLeaks have adamantly denied they were in contact.

However, prosecutors are closely examining both public comments and alleged private assertions that Stone made in 2016 suggesting he had a way to reach Assange, the people said.

Last month, Randy Credico, a onetime Stone friend, told the grand jury that the Trump loyalist confided during the 2016 campaign that he had a secret back channel to WikiLeaks, according to a person familiar with the matter.