The special counsel’s report confirmed this week that Seth Rich, a young Democratic National Committee employee whose unsolved killing became grist for a right-wing conspiracy theory, was not the source of thousands of internal D.N.C. emails that WikiLeaks released during the 2016 presidential race, officially debunking a notion that had persisted without support for years.

Tucked amid hundreds of pages of the report’s main findings, the special counsel, Robert S. Mueller III, took aim at WikiLeaks and its founder, Julian Assange, for falsely implying that Mr. Rich was somehow involved in the dissemination of the emails, an act that aided President Trump’s campaign.

“WikiLeaks and Assange made several public statements apparently designed to obscure the source of the materials that WikiLeaks was releasing,” according to the report, which showed that WikiLeaks corresponded with the true source of the leaked emails — Russian hackers — after Mr. Rich’s death.

The confirmation comes after years of anguish for Mr. Rich’s family, who fought attempts to politicize and spread misinformation about his killing, which is believed to have happened during a bungled robbery attempt.