The FBI is hunting a 12-man terrorist "sleeper cell" linked to the brothers suspected of carrying out the Boston bombings.
The Bureau believes that Tamerlan and Dzhokhar Tsarnaev received training in bomb-making before they carried out the attack, which killed three people.
It emerged 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.
A source close to the investigation said: "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."
More than 1,000 FBI agents have been assigned to track down the cell. A specialist team of CIA and FBI interrogators is on hand at Beth Israel Deaconess Medical Centre in Boston, waiting to question Dzhokhar.
The FBI has admitted interviewing Tamerlan two years ago, at the request of the Russian government, but said it wound up its inquiries after interviews with Tamerlan and other family members "did not find any terrorism activity, domestic or foreign".
The FBI now says the pair were part of a terrorist cell, though they were not under surveillance at the time of the bombings, US Government officials said.
The brothers were carrying six bombs, a handgun and a rifle at the time of the shoot-out in which Tamerlan was killed. Three devices - thought to be pipe bombs - were detonated.
Investigators are working to establish whether the brothers had help in making the bombs, which were contained in pressure cookers and packed with nails and ball bearings.