WASHINGTON — Jerome Corsi, a conspiracy theorist and ally of the former Trump campaign adviser Roger J. Stone Jr., is in plea negotiations with prosecutors working on the special counsel investigation into Russia’s interference in the 2016 election.

A person familiar with the talks said that the prosecutors had presented Mr. Corsi with evidence that he had not been truthful when investigators asked him whether he knew beforehand that WikiLeaks was going to publish emails stolen from Democratic computers during the campaign.

The special counsel, Robert S. Mueller III, and his team are trying to discover whether anyone connected with the Trump campaign knew of or cooperated with the Russian intelligence operatives who hacked the Democratic systems and funneled the emails to WikiLeaks in an attempt to undercut Hillary Clinton’s presidential campaign.

Although Mr. Corsi apparently had no direct connection to the Trump campaign, he was in touch with Mr. Stone, a former campaign adviser who communicated with senior campaign officials through the election. Mr. Stone claimed during the campaign that he had a back channel to WikiLeaks, but now says he was merely bluffing to unsettle Mrs. Clinton’s team. Mr. Stone also communicated with Guccifer 2.0, the online persona used by one or more Russian intelligence operatives who hacked the Democratic systems.