The US Department of Homeland Security has launched an internal investigation into an attempt from one of its agencies to unmask the identities of people behind an anti-Trump Twitter account. As The Wall Street Journal reports, the investigation was confirmed in a letter from DHS Homeland Inspector General John Roth to Senator Ron Wyden.

The investigation stems from a March 14th summons sent by US Customs and Border Protection (CBP) to Twitter. The summons requested information that could have been used to identify the operator(s) of @alt_USCIS, a “rogue” Twitter account that purported to reflect the views of a dissenting Citizenship and Immigration Services employee. Several rogue Twitter accounts sprang up following this year’s presidential inauguration, with several expressing anti-Trump criticism.

Twitter challenged the order in a federal lawsuit in April, questioning the Customs agency’s legal authority to request such information. The lawsuit also warned that identifying the user(s) behind @alt_USCIS “would have a grave chilling effect on the speech of that account in particular and on the many other ‘alternative agency’ accounts that have been created to voice dissent to government policies.” The DHS rescinded the summons one day after Twitter filed the lawsuit, which was subsequently withdrawn.

In his letter to Sen. Wyden, who earlier requested an internal review, Roth said his office is investigating “whether CBP abused its authority in issuing the March 14, 2017 summons to Twitter.” He added that the investigation will review “potential broader misuse of summons authority at the Department and/or its components.”

The letter also notes that the inspector general’s office “has not played any role in attempting to identify the owner of the @alt_USCIS Twitter account,” and that the internal review would not investigate “any alleged misconduct” on the part of the account’s operators. “We strive to ensure that our work does not have a chilling effect on individuals’ free speech rights,” Roth said.