Depends on your definition of a terrorist cell. I think that can be just one person that is promoting and recruiting others to terrorize for whatever their cause. We know that one of the dead terrorists has been watched and known by our government to be a recruiter - was it for Al Qaeda? I can't remember - but this guy may have recruited the other one that was shot along with him. If he managed to convince one person to join him in jihad - who is to say that there aren't others he was working with as well?
I don't understand the rush to label these guys as 'lone wolves' - like that makes it less terrible.
And besides - how many lone wolves does it take to make a pack? 
There's a visible difference between enraging a couple of fringe nutjobs to the degree that they'll decide to go "defend the Prophet" and activating a trained and prepared cell like Tamerlan Tsarnaev and his brother. That difference is in the results of the attacks.
The Garland shooters were taken down in about 15 seconds.
So the definition of a terrorist cell (there is only one definition BTW) includes training. Tsarnaev reaceived months of training, but there's no evidence that these guys did.
