Reports indicate the FBI is searching for a 12-person "sleeper cell" tied to the Boston Marathon bombers.This is an outworking of the belief that "Tamerlan and Dzhokhar Tsarnaev were "specially trained" to carry out their attack.
The UK's Daily Mirror quoted an anonymous source with knowledge of the investigation:
We have no doubt the brothers were not acting alone. The devices used to detonate the two bombs were highly sophisticated and not the kind of thing people learn from Google.
...Someone gave the brothers the skills and it is now our job to find out just who they were. Agents think the sleeper cell has up to a dozen members and has been waiting several years for their day to come. (Continues)