The Office of the Inspector General of the Department of Homeland Security has launched an internal investigation into federal efforts to “unmask” the owner of a Twitter account critical of immigration policies.

Inspector General John Roth informed Sen. Ron Wyden (D-Ore.) in a letter Friday that his office is conducting a probe to determine whether behavior by federal officials concerning the Twitter account was “improper in any way” or if there was an “abuse of authority.”

U.S. Customs and Border Protection, which is overseen by the Department of Homeland Security, last month served Twitter with a summons demanding the identity of the operator of the account for “Alt Immigration” – @ALT_uscis. The account is one of dozens that sprang up after Donald Trump’s inauguration purporting to be the rebellious voice of truth from within various federal agencies or from agency supporters.

Twitter in turn sued CBP and DHS to block the action, and the demand was dropped.

We're glad Twitter is pushing back. We'll be going to court to defend this user's right to anonymous speech. https://t.co/tqj5XrNvgn — ACLU National (@ACLU) April 6, 2017

Wyden, who called the action against Twitter a “witch hunt,” complained in a letter to the CBP commissioner that he was concerned the summons represented a “disturbing threat to free speech and whistleblower protections” — and he called for an investigation.

Roth, who received a copy of Wyden’s letter, wrote to the senator that DHS had determined, at CBP’s request, that no classified information was released via the @ALT_uscis Twitter account. Roth said his own office was unaware of the effort to unmask the identity of the Twitter account holder until officials read about it in the media.

“We strive to ensure that our work does not have a chilling effect on individuals’ free speech rights,” wrote Roth, who emphasized that his office “is not ... and will not investigate, any alleged misconduct on the part of the @ALT_uscis account owner relating to his or her use of the Twitter account.”

His office is, however, investigating whether CBP’s conduct concerning the Twitter account was “improper in any way, including whether CBP abused its authority in issuing the .... summons to Twitter.” Roth added that his office is “also reviewing potential broader misuse of summons authority.”

Wyden could not immediately be reached for comment.

We want to thank @twitter and @aclu for standing up for the right of free anonymous speech. Thank you resistance for standing up for us. https://t.co/6PdwZIJ2xP — ALT🛂 Immigration (@ALT_uscis) April 7, 2017

CORRECTION: A previous version of this article stated incorrectly that U.S. Citizen and Immigration Services is part of Customs and Border Protection; rather, they are separate components within the Department of Homeland Security.