(CNN) Special counsel Robert Mueller asked a government agency last June to preserve documents relating to Donald Trump's transition to the presidency, according to records obtained by CNN -- an indication of how he expanded the investigation soon after his appointment.

The formal preservation request to the General Services Administration, the agency that supports presidential transitions, was sent on June 22, about a month after Mueller was named special counsel.

An email from March 2017 between the FBI and GSA -- months before Mueller was appointed -- suggests FBI investigators' interests at that time were narrower. Then the FBI asked GSA to consult with lawmakers before disposing of other transition documents.

The more expansive request came when an agent in the FBI's counterintelligence division emailed the deputy general counsel at GSA to preserve documents, electronics and communications from the Trump transition team, according to documents CNN obtained through a Freedom of Information Act request.

"As a follow up to our request, we thought it best to send GSA an official preservation letter. Your staff has been extraordinarily responsive and we appreciate their patience with our (my) questions," the agent wrote, according to the email obtained by CNN. GSA redacted the name of the FBI agent who sent the email.

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