Mr. Stone has repeatedly said that he had access only to Mr. Assange’s public statements and to secondhand information from journalists or other sources. If he implied that he had more direct sources, he has said, he was simply engaging in political hyperbole.

In an interview on Wednesday, Mr. Stone insisted he did nothing more than “posture, bluff, hype,” based on WikiLeaks’ Twitter feed and miscellaneous tips.

“I didn’t need any inside knowledge to do that. They keep looking for some direct communication with WikiLeaks that doesn’t exist,” he said of journalists reporting on the Russia investigation. He added that he had paid for two polygraph tests that prove he is telling the truth.

But some of his previous statements have proved false. Mr. Stone told The Washington Post this week that he never discussed WikiLeaks with Trump campaign officials. “There are no such communications and if Bannon says there are he would be dissembling,” he said.

This article is based on interviews with people familiar with the Russia investigation and the inner workings of the Trump campaign, as well as a review of hundreds of text messages and emails that Mr. Stone exchanged over months with several associates, including Randy Credico, a New York comedian, former radio host and left-wing activist whom Mr. Stone has repeatedly identified as his source about WikiLeaks.

Besides the confusion caused by Mr. Stone’s penchant for innuendo and outright lies, investigators are hampered by the fact that Mr. Assange remains out of reach, holed up in the Ecuadorean Embassy in London, where he has lived for six years in fear of extradition to face possible criminal charges.

Still, Mr. Bannon’s October 2016 email correspondence shows that the perception that Mr. Stone knew what WikiLeaks had in store for Mrs. Clinton spread to the highest levels of the Trump campaign. No evidence has emerged that Mr. Trump or his advisers alerted the authorities.