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, US officials said.
The two ethnic Chechen brothers practised Islam. One died in a police shootout on Friday, and the other was formally charged on Monday after being questioned by federal officials in his hospital room where he is recovering from multiple injuries.
Dzhokhar Tsarnaev, 19, was charged with using a weapon of mass destruction to kill - a crime that carries a possible death sentence.
A Boston mosque where the two sometimes attended services said one of them had outbursts during two sermons that encouraged Muslims to celebrate American institutions such as the Fourth of July and figures such as Martin Luther King.
The Islamic Society of Boston Cultural Centre said Tamerlan Tsarnaev, 26, argued with a preacher during a service at its Cambridge mosque in November about US holidays. The centre said he returned to services and in January called an elder a "hypocrite" for praising Martin Luther King. The centre says congregants shouted at him, telling him to leave. The mosque says there were no further incidents after that, even as the elder Tsarnaev continued attending prayers and services.
Officials have said the brothers set off two pressure-cooker bombs at last week's race that sprayed shrapnel into the crowds, killing three people and wounding more than 200. The younger brother was listed in serious but stable condition at Beth Israel Deaconess Medical Centre, unable to speak because of a gunshot wound to the throat.
In a criminal complaint outlining the evidence, the FBI said he was seen on surveillance cameras putting a rucksack on the ground near the site of the second blast and then manipulating a mobile phone and lifting it to his ear. The charges shed no light on a motive for the attack.
After the first explosion ripped through the crowd, a calm-looking Tsarnaev quickly walked away, and about 10 seconds later, the second blast occurred where he left the rucksack, the FBI said. The FBI did not make it clear whether authorities believe he used his phone to detonate one or both of the bombs or whether he was talking to someone. The court papers also said that during the police pursuit that led to the older brother's death and the younger one's capture, one of them told a carjacking victim: "Did you hear about the Boston explosion? I did that."
Tsarnaev was charged with using and conspiring to use a weapon of mass destruction against persons and property, resulting in death. He is also likely to face state charges in connection with the shooting death of a university police officer. The Obama administration said it had no choice but to prosecute Tsarnaev in the federal court system.
On Monday, 51 people remained in hospital, three in critical condition and five listed as serious. At least 14 people lost all or part of a limb; three of them lost more than one. Two children with leg injuries remain at Boston Children's Hospital. A seven-year-old girl is in critical condition and an 11-year-old boy is in fair condition.