The threat group known as The Dark Overlord has claimed responsibility for a law firm data breach involving files allegedly related to the 9/11 terrorist attacks.

The Dark Overlord first announced on New Year’s Eve that it had stolen files belonging to Llyod’s of London, Silverstein Properties and Hiscox Syndicates Ltd., according to Motherboard. Although the group’s announcement on the Pastebin messaging service has been deleted, Motherboard confirmed the hack with Hiscox.

The stolen information reportedly includes email and voicemail messages as well as legal files such as non-disclosure strategies and expert witness testimonies.

9/11 Data Held for Ransom

In a Dec. 31 tweet, The Dark Overlord claimed it had managed to steal more than 18,000 secret documents that would provide answers about 9/11 conspiracy theories. Twitter has since suspended the group’s account.

SC Magazine reported that the law firm paid an initial ransom, but then violated terms of agreement by reporting the incident to law enforcement. The threat group is now demanding a second ransom be paid in bitcoin and said it will also sell information obtained in the breach to interested third parties on the dark web.

According to a post on Engadget, The Dark Overlord also attempted to prove it had committed the data breach by publishing nonsensitive material from other law firms as well as organizations such as the U.S. Transportation Security Administration (TSA) and Federal Aviation Authority (FAA).

How to Limit the Threat of Groups Like The Dark Overlord

This latest attack from The Dark Overlord is further proof that data breaches can not only create a PR nightmare, but also put organizations’ survival and, in some cases, national security at risk.

Unfortunately, the exact details around how The Dark Overload accessed the law firm’s network are unknown. Security experts recommend conducting a short but comprehensive 15-minute self-assessment to gauge the organization’s IT security strengths and weaknesses. The results can be benchmarked against similar firms, and security leaders can gain access to the expertise they need to keep groups like The Dark Overlord away from their data.