Peter Navarro, a top White House adviser to Donald Trump, is being commanded by a federal grand jury subpoena to turn over to the justice department his communications with the former president, the former president’s attorneys and the former president’s representatives.

The exact nature of the subpoena – served on 26 May 2022 and first obtained by the Guardian – and whether it means Trump himself is under criminal investigation for January 6 could not be established given the unusually sparse details included on the order.

But certain elements appear to suggest that it is related to a new investigation examining potential criminality by the former president and, at the very least, that the justice department is expanding its inquiry for the first time into Trump and his inner circle.

The subpoena compelled Navarro to either testify to a grand jury early next month, or produce to prosecutors all documents requested in a separate congressional subpoena issued earlier this year by the House select committee investigating the January 6 Capitol attack.

“All documents relating to the subpoena dated February 9, 2022, that you received from the House select committee,” the justice department says in the subpoena, “including but not limited to any communication with former President Trump and/or his counsel or representatives.”

The existence of the federal subpoena was revealed in a lawsuit filed by Navarro that sought to declare the congressional subpoena unlawful.

It remains entirely possible, given the explicit reference to the select committee, that the grand jury subpoena indicates the US attorney for the District of Columbia, Matthew Graves, is building a criminal contempt of Congress charge against Navarro.

The assistant US attorney listed on the subpoena, Elizabeth Aloi, is also listed as working in the office of the US Attorney for the District of Columbia that handles contempt of Congress cases – though that is not necessarily indicative of the kind of investigation involved.

The confounding aspect of this grand jury subpoena, according to three former assistant US attorneys who spoke on the condition of anonymity, is that targets of investigations are rarely subpoenaed. And “process” charges such as contempt do not require subpoenas for documents.

But the fact that Trump is specifically named in the subpoena – a reference that the justice department would not have made lightly – and the specific requests for Navarro’s communications with Trump could indicate that this is a criminal investigation examining Trump.

The internal US attorney’s office number and the ID number of the grand jury subpoena to Navarro suggests that the investigation is a new line of inquiry for the justice department. Variants of #GJ2022052590979 or USAO #2022R00631 have not surfaced on other subpoenas.

At least four separate grand juries are now examining events related to the January 6 Capitol attack.

One grand jury was impaneled last year for a contempt charge against Trump’s strategist Steve Bannon. A second is examining organizers of pro-Trump rallies, a third is looking at Trump lawyers in a scheme to falsify slates of electors, and now a fourth concerns Navarro.

Navarro was not told when he was served with the grand jury subpoena whether he was a target or a subject of the investigation. If he was a target, that might indicate the subpoena was related to a contempt case. If he was a subject, it could make him part of a wider inquiry.

The distinction also raises a third possibility, according to the former assistant US attorneys: he may be a target for a contempt case, and also a subject in a different case – and prosecutors might use the contempt case as leverage to gain cooperation for the other.

A spokesman for the justice department and the US attorney’s office did not respond to requests for comment.

In his lawsuit, Navarro is challenging both the validity of the congressional subpoena as well as the federal grand jury subpoena. Navarro argues the federal grand jury subpoena is invalid since it requests materials demanded in the congressional subpoena, which he argues is also invalid.

“The US Attorney cannot issue a Grand Jury Subpoena deemed to be lawful and enforceable that is derivative of a fruit of the poisonous tree ultra vires, unlawful, and unenforceable subpoena issued by the Committee,” Navarro writes in the 88-page filing.

Navarro also contends that by demanding his communications with Trump, the justice department is improperly asking him to violate executive privilege – privilege that he says has not been waived by the former president.