Two US officials say preliminary evidence from an interrogation suggests the suspects in the Boston Marathon attack were motivated by religion but were apparently not tied to any Islamic terrorist groups.
The two brothers, ethnic Chechens, practiced Islam.
The US officials spoke on Monday on condition of anonymity because they were not authorised to publicly discuss the investigation.
One of the brothers died in a police shootout on Friday. The other brother was formally charged on Monday after being questioned by federal officials in his hospital room where he is recovering from multiple injuries.