Cell
A group of terrorists (in this case Al qaeda) who plant themselves in a region, develop roots and remain inactive (sleeping) awaiting the order to strike. Once they have received a signal to act - they become "activated". Each may have different missions.
Or as Tom pointed out, it could also be copycat sympathisers working on their own to create terror.
However, if you note the timing - it occurred as another terrorist cell in Seattle was rounded up. There is a good chance they thought they might be next and activated before they were revealed and captured.
SDAI-Tech1