Special Counsel Robert Mueller caught the Trump campaign by surprise last month, issuing a subpoena to more than a dozen officials despite the campaign's voluntary compliance with his probe, the Wall Street Journal reported Thursday, citing a source.
The Journal called the subpoena the "first official order" for information from the campaign. The campaign is responding to the order on an "ongoing" basis, the source reportedly told the outlet.
None of those who received the subpoena were compelled to testify before a grand jury, the Journal reported.
"Sending a subpoena to an entity that says it has been cooperating with document requests isn't unusual in cases in which prosecutors have some concern that their demands aren't being met promptly or aren't being entirely fulfilled," the Journal wrote, citing former prosecutors.
Mueller is overseeing an investigation into attempts by the Russian government to meddle in the 2016 election and potential collusion with Donald Trump's presidential campaign.
Neither the White House nor a spokesperson for the special counsel responded immediately to a request for comment Thursday night. Trump has previously denied working with the Russian government for political benefit.
The special counsel's office declined to comment when contacted by CNBC about the Journal's report. A representative for the White House also declined to comment.