Lawyers for Ross Ulbricht, the alleged mastermind behind Silk Road, suffered a setback Tuesday, after a federal judge limited their ability to pursue a strategy of blaming someone else for the operation of the online drug marketplace.

U.S. District Judge Katherine Forrest ruled on Tuesday that Mr. Ulbricht’s lawyers are no longer allowed to ask government witnesses whether they “believed” or “suspected” Mark Karpeles to be the real operator of Silk Road, calling those questions “irrelevant” to the trial. Instead, Mr. Ulbricht’s lawyers can only present facts about Mr. Karpeles that come from witnesses’ firsthand knowledge, not from witnesses getting information from secondhand sources, the judge said.

Mr. Ulbricht’s lawyer, Joshua Dratel, revealed last week that he plans to prove his client was framed by Mr. Karpeles, who was the head of the now-bankrupt bitcoin exchange Mt. Gox.

The prosecutors’ objections arose during Mr. Dratel’s cross-examination of Jared Der-Yeghiayan, the government’s first witness who was a special agent with the Department of Homeland Security. Mr. Der-Yehighiayan had said he began investigating Mr. Karpeles because a website linked to Silk Road used the same software platform as a website run by Mr. Karpeles.

The government hasn't accused Mr. Karpeles of any wrongdoing in relation to Silk Road and one his lawyers has denied the accusation that Mr. Karpeles ran the site.