Boston marathon attack
Boston marathon attackReuters

British tabloid newspaper The Mirror says that police believe Tamerlan and Dzhokhar Tsarnaev were specially trained to carry out the devastating attack, and were part of a larger "sleeper cell." This report is not corroborated by other news outlets and its veracity cannot be ascertained.

The Mirror quotes a source close to the investigation who told it: “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.

“They were too advanced. 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.”

A specialist team of CIA and FBI interrogators was flown Saturday to Boston's Beth Israel hospital to interrogate the younger brother, Dzhokhar. He is alive but in serious condition and reportedly is in no condition to talk to investigators.

The report added that "a man and two women were arrested 60 miles from Boston in the port of New Bedford in the hours before Dzhokhar's capture. The three are believed to be of college age."

The FBI are retracing elder brother Tamerlan's movements during a six-month trip he made to the Russia Caucusus last year, including a visit to Chechnya. "It's a key thread for investigators," Kevin Brock, a senior FBI counter-terrorism official, told The Mirror.

Tamerlan's Russian-language YouTube page contains numerous links to extremist Islamic sites, including one that features an hour-long speech by the radical cleric Feiz Mohammed, along with other videos labeled "Terrorists" and "Islam".