Investigators have been trying to determine whether Tsarnaev's older brother, Tameran who was killed while the two were on the run after the bombing, was influenced or trained by Islamic militants during a trip overseas. But the indictment released Thursday against 19-year-old Dzhokhar makes no mention of any overseas influence.
Before the attack, according to the indictment, he downloaded 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, explosive powder extracted from fireworks, and lethal shrapnel.
He also downloaded extremist Muslim literature, including "Defense of the Muslim Lands, the First Obligation After Imam," which advocates "violence designed to terrorize the perceived enemies of Islam," the indictment said. The article was written by the late Abdullah Azzam, whose legacy has inspired terrorist attacks in the Middle East.
Another tract downloaded - titled "The Slicing Sword, Against the One Who Forms Allegiances With the Disbelievers and Takes Them as Supporters Instead of Allah, His Messenger and the Believers" - included a foreword by Anwar al-Awlaki, an American propagandist for al-Qaida who was killed in a U.S. drone strike in 2011.
The 30-count 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.
"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 at a news conference with federal prosecutors on Thursday.
The indictment contains the bombing charges, punishable by the death penalty, that were brought in April against Tsarnaev, including use of a weapon of mass destruction to kill. It also contains many new charges covering the slaying of an MIT police officer and the carjacking of a motorist during the getaway attempt that left Tamerlan Tsarnaev dead.
U.S. Attorney Carmen Ortiz of Massachusetts said Attorney General Eric Holder will decide whether to pursue the death penalty against Tsarnaev, who will be arraigned on July 10.
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 backyard in Watertown, Mass.
According to the indictment, he scrawled messages on the inside of the vessel that said, among other things, "The U.S. Government is killing our innocent civilians," ''I can't stand to see such evil go unpunished," and "We Muslims are one body, you hurt one you hurt us all."
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 U.S. about a decade.
There was no mention in the indictment of any larger conspiracy beyond the brothers, and no reference to any direct overseas contacts with extremists. Instead, the indictment suggests the Internet played an important role in the suspects' radicalization.
Tamerlan Tsarnaev spent six months in Dagestan last year, and investigators traveled to the Russian province to talk to the men's parents and try to determine whether he was influenced or trained by local Islamic militants.
Christina DiIorio-Sterling, a spokeswoman for Ortiz, declined to comment on why the indictment did not mention whether authorities believe the elder Tsarnaev received any training during his stay in Russia.
The indictment assembled and confirmed details of the case that have been widely reported over the past two months, and added new pieces of information.
For example, it corroborated reports that Tamerlan Tsarnaev bought 48 mortar shells from a Seabrook, N.H., fireworks store. It also disclosed that he used the Internet to order electronic components that could be used in making bombs.
The papers detail how the brothers then allegedly placed knapsacks containing shrapnel-packed bombs near the finish line of the 26.2-mile race.
The court papers also corroborated reports by authorities that Dzhokhar Tsarnaev contributed to his brother's death by accidentally running him over with a stolen vehicle during a shootout and police chase.
The charges cover the slaying of Massachusetts Institute of Technology police officer Sean Collier, who authorities said was shot in the head at close range in his cruiser by the Tsarnaevs, who tried to take his gun.
In addition, prosecutors said that during the carjacking, the Tsarnaevs forced the motorist to turn over his ATM card and his password, and Dzhokhar withdrew $800 from the man's account.
At the same time the federal indictment was announced, Massachusetts authorities brought a 15-count state indictment against Dzhokhar over the MIT officer's slaying and the police shootout.