The news was first reported by Axios. Unnamed sources within the transition team told the news site that the Trump team realized Mueller had obtained the emails when they were used as the basis of questions put to witnesses. The news has since been independently verified.

The emails could prove crucial to the ongoing investigation. According to Axios’ sources, they include discussions of appointments in the new administration, as well as discussion of press strategy and policy planning. The biggest question looming over Mueller’s investigation is whether Russia’s interference in the 2016 election, which has been confirmed by U.S. intelligence agencies, was coordinated in any way with the Trump campaign.

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Kory Langhofer, a lawyer for the Trump transition, alleged in a letter Saturday that the emails were obtained unlawfully, and violated Fourth Amendment protections against unreasonable search and seizure. They were reportedly provided to Mueller directly by staff at the General Services Administration, which hosted the transition email accounts. Langhofer claims that the trove includes “private materials” and material “subject to privilege claims.”

That may be a reference to either attorney-client privilege, executive privilege protecting the President, or both. Claims of executive privilege would likely be subject to legal challenge in the case, since the doctrine is intended primarily to protect national security — the overarching purpose of Mueller’s investigation.

A spokesman for the special counsel’s office told Reuters that all emails obtained during the investigation were collected with “either the account owner’s consent or appropriate criminal process.”