Boston Marathon suspect Dzhokhar Tsarnaev downloaded bomb-making instructions from an al Qaida magazine, legal documents have revealed.
He also gathered online material on Islamic jihad and martyrdom and later scrawled anti-American messages inside the boat where he lay wounded, a charge sheet alleged.
The 30-count indictment contains the bombing charges, punishable by the death penalty, that were brought in April against the 19-year-old Tsarnaev, including use of a weapon of mass destruction to kill. It also contains many new charges covering the killing of an Massachusetts Institute of Technology police officer and the carjacking of a motorist during the getaway attempt that left Tsarnaev's older brother, Tamerlan, dead.
"Tamerlan Tsarnaev's justice will be in the next world, but for his brother, accountability will begin right here in the district of Massachusetts," Suffolk District Attorney Daniel Conley, whose jurisdiction includes Boston, said.
The indictment provides one of the most detailed public explanations to date of the brothers' alleged motive - Islamic extremism - and the role the internet may have played in influencing them.
Three people were killed and more than 260 wounded by the two pressure-cooker bombs that went off near the finish line of the marathon on April 15. Dzhokhar Tsarnaev was captured four days later, hiding in a boat parked in a back garden in Watertown, just west of Boston. According to the document, he scrawled messages on the inside of the vessel including "The US Government is killing our innocent civilians".
The Tsarnaev brothers had roots in the turbulent Russian regions of Dagestan and Chechnya, which have become recruiting grounds for Muslim extremists. They had been living in the US for about a decade.
But the indictment made no mention of any larger conspiracy beyond the brothers and no reference to any direct overseas contacts with extremists. Instead, it suggests the internet played an important role in the suspects' radicalisation.
Before the attack, according to the document, Dzhokhar Tsarnaev downloaded onto his computer the summer 2010 issue of Inspire, an online English-language magazine published by al Qaida. The issue detailed how to make bombs from pressure cookers, and explosive powder extracted from fireworks and lethal shrapnel, and he also downloaded extremist Muslim literature.
Attorney General Eric Holder will decide whether to pursue the death penalty against Tsarnaev. Some of Tsarnaev's relatives have insisted he is not guilty, and his mother has said the allegations against him and his brother are lies.