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.