The Sleeper Cell was being tracked by Erica as a possible terrorist sleeper cell. Among all her other subjects, this group alone was active during the arrival of the Visitors, attracting her special attention.
Prior to their arrival on earth, the Visitors sent an advance group. They have assimilated into the human environment, obtaining careers in political, religious, business, and military fields. Some, if not most, have organized themselves in a Sleeper Cell, attracting the attention of the FBI for their activities in trafficking drugs and weapons, and counterfeiting documents such as passports.