You may be looking for Section 31, the rogue Federation agency.

Sector 31 is a region of Federation controlled space located near the Romulan Neutral Zone. In 2364, communication with Starbases in this sector, and outposts in neighboring Sector 30 ceased as of stardate 41903.2. The facilities are later determined as destroyed by an unknown assailant. (TNG: "The Neutral Zone") It is later determined that the assaliant was the Borg. (TNG: "Q Who")