The two brothers suspected of bombing the Boston Marathon appear to have been motivated by a radical brand of Islam but do not seem connected to any Muslim terrorist groups.
The comments from US officials came after interrogating and charging Dzhokhar Tsarnaev with crimes that could bring the death penalty.
19-year-old Tsarnaev was charged in his hospital room, where he is still in serious condition with a gunshot wound to the throat and other injuries suffered during his attempted getaway.
Among those terribly injured by the Boston marathon bombs was a dance instructor. Adrianne Haslet-Davis lost her foot in the explosion.
The mother of the two brothers suspected of carrying out the Boston bombings told ITV News they saw last year's race but are innocent.
Boston's marathon bombing suspect Dzohkhar Tsarnaev was charged with an offence that could lead to the death penalty, if he is found guilty.