ABC News also reports that the brothers are believed to have learned how to make the deadly pressure cooker bombs on the internet.
Meantime, Dzhokhar Tsarnaev, 19, has been charged with using a weapon of mass destruction resulting in the death of three people and injuries to more than 200 people at the Boston Marathon.
Tsarnaev remains hospitalized in serious but stable condition with wounds to the neck and throat.
He made an initial court appearance in his hospital room.
Tsarnaev, will not be tried as an enemy combatant, the White House said today. Tsarnaev is a naturalized U.S. citizen, and under U.S. law, American citizens cannot be tried by military tribunals, White House spokesman Jay Carney said.
"We will prosecute this terrorist through our civilian system of justice," Carney told reporters. "The system has repeatedly proven that it can successfully handle the threat that we continue to face," he said, citing Times Square bomber Faisal Shahzad, underwear bomber Umar Farouk Abdulmutallab and others as examples.
Tsarnaev is specifically charged with one count of using and conspiring to use a weapon of mass destruction (namely, an improvised explosive device or IED) against persons and property within the United States resulting in death, and one count of malicious destruction of property by means of an explosive device resulting in death. The statutory charges authorize a penalty, upon conviction, of death or imprisonment for life or any term of years.
Dzhokhar Tsarnaev's brother, 26-year-old Tamerlin Tsarnaev, was killed in a shootout with police early last Friday morning.
A report out in the Wall Street Journal says just before 26-year-old Tamerlin Tsarnaev was killed, he called his mother in Russia. He reportedly said, "The police, they have started shooting at us. They are chasing us. Mama, I love you." then his mother says the phone went silent.
In outlining the evidence against him in court papers, the FBI said Tsarnaev was seen on surveillance cameras putting a knapsack down on the ground near the site of the second blast and then manipulating a cellphone and lifting it to his ear.
After the first explosion went off about a block down the street and spread fear through the crowd, Tsarnaev - unlike nearly everyone around him - looked calm and quickly walked away, the FBI said. Just 10 seconds or so later, the second blast occurred where he left the knapsack, the FBI said.
The FBI did not make it clear whether authorities believe he used his cellphone to detonate one or both of the bombs or whether he was talking to someone.
The court papers also said that during the long night of crime Thursday and Friday that led to the older brother's death and the younger one's capture, one of the Tsarnaev brothers told a carjacking victim: "Did you hear about the Boston explosion? I did that."
Seven days after the bombings, meanwhile, Boston was bustling Monday, with runners hitting the pavement, children walking to school and enough cars clogging the streets to make the morning commute feel almost back to normal.
Residents paused in the afternoon to observe a moment of silence at 2:50 p.m., the time of the first blast. Church bells tolled across the city and state in tribute to the victims.
Also, hundreds of family and friends packed a church in Medford for the funeral of bombing victim Krystle Campbell, a 29-year-old restaurant worker. A memorial service was scheduled for Monday night at Boston University for 23-year-old Lu Lingzi, a graduate student from China.
Fifty-one victims remained hospitalized Monday, three of them in critical condition.
Federal authorities went to a Rhode Island home belonging to the parents of Katherine Tsarnaev. She's the widow of Tamerlan Tsarnaev, and she and their 3-year-old are reportedly staying with her parents. Her lawyer did the talking, and apparently her family has put their house up for sale.
The six-block area of Boylston Street where the bombs went off remains closed, but once the FBI releases the scene, the city will follow a five-step process to reopen the area. It's still unclear when that will be.
After the two brothers engaged in a gun battle with police early Friday, authorities found many unexploded homemade bombs at the scene, along with more than 250 rounds of ammunition.
Police Commissioner Ed Davis said the stockpile was "as dangerous as it gets in urban policing."
"We have reason to believe, based upon the evidence that was found at that scene - the explosions, the explosive ordnance that was unexploded and the firepower that they had - that they were going to attack other individuals," he said. "That's my belief at this point."
Pictures from the Boston Marathon bombings (The Associated Press contributed to this report)