Details of more than 40,000 Department of Homeland Security (DHS) employees have been leaked to the public.
Data transparency group Distributed Denial of Secrets said on Wednesday it had released the data obtained by an entity calling itself the Department of Peace.
The information included details about officials working in sensitive fields such as surveillance.
Names, phone numbers and email addresses of employees from various agencies, including Immigration and Customs Enforcement (ICE), were included in the data.
Job titles and email groups were also released, which, among other details, showed which agents were assigned to special operations, surveillance jobs, computer forensics and undercover work.
DHS said in a statement it was "aware of a potential breach" and was "actively working to address the situation swiftly and effectively".
It added that it was working to ensure its employees were properly protected and would update the public "when more information is available".
In a statement, Distributed Denial of Secrets co-founder Emma Best said the hacktivist and journalist known as "maia arson crimew" developed a search tool for the data that would allow the public to browse it.
She added that the tool would "replace anonymity with accountability" and empower journalists, guide public records requests and boost congressional oversight.
The Department of Peace said in a statement that DHS employees "have our information, so it's only fair that we have theirs".
It said it accessed the DHS data through the sign-on service, Okta.
Okta told the Reuters news agency: "Okta is secure, and we have no evidence our data or systems were compromised."
In March, the Department of Peace claimed responsibility for hacking the DHS and posting documents related to contracts with outside vendors.
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It is the third major government data leak to have made headlines in the past month.
In September, the hacking group ShinyHunters claimed the theft of a massive trove of data from the FBI, including sensitive job and medical details from thousands of employees.
Last week, the US defence department was hit with a lawsuit following a breach that exposed data on millions of current and former military personnel.
(c) Sky News 2026: Details of more than 40,000 Homeland Security workers leaked in major hack
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