Counsel for Trump for America Kory Langhofer told ABC News in a statement Saturday, "This morning we sent a letter to the House and the Senate concerning unauthorized disclosure of private and privileged transition emails with the special counsel's office."
Langhofer said the emails that he alleges were illegally provided to Mueller involve 13 transition staff, including four senior officials.
The Trump transition legal team claims the disclosure of the emails amounts to "unlawful conduct" as a release of "privileged communications" of nongovernment officials, and that it breaks with years of legal precedent around preserving the privacy of presidential transition teams.
The GSA, which assisted the Trump team with administrative functions during the transition to office, including hosting its email services -- did not respond to ABC News' request for comment.
A spokesman for the special counsel neither confirmed nor denied Langhofer's allegations, but said in a statement to ABC News, "When we have obtained emails in the course of our ongoing criminal investigation, we have secured either the account owner’s consent or appropriate criminal process."
A House Oversight Committee spokesperson told ABC News Sunday that it had received Langhofer's letter, but that the handling of his inquiry would be best addressed in the court system.
"The central issues raised are fact specific legal issues which involve issues of privilege, waiver (express, implied, actual and constructive), standing to assert claims of privacy, expectations of privacy and the reasonableness thereof, third party consent, statutory constructions, and inevitable discovery among other issues," the spokesperson said in a statement. "These are issues to be briefed by the parties (or others with cognizable legal claims and standing) and decided by the court — not Congress."
The content of the emails or why Mueller could be interested in them is unclear. The special counsel has been investigating contacts between some former Trump campaign officials and Russians in his probe to determine whether any crimes were committed.
ABC News' Mike Levine and Devin Dwyer contributed to this report.