The United States Federal Bureau of Investigation is hunting for a 12-strong terrorist 'sleeper cell' linked to the Boston marathon bomb brothers.
Police believe Tamerlan and Dzhokhar Tsarnaev were specially trained to carry out the devastating attack.
According to the Mirror, more than 1,000 FBI agents are working to track down the cell and arrested a man and two women 60 miles from Boston.
A source close to the investigation said that they 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.
The source said that they were too advanced. Someone gave the brothers the skills and it is now their job to find out just who they were.
It added that the agents think the sleeper cell has up to a dozen members and has been waiting several years for their day to come.
According to the report, a specialist team of CIA and FBI interrogators has also flown to a Boston hospital to grill wounded Dzhokhar, 19, about the secret group, the report added.
