Sources say the brothers surprised themselves at how quickly they could build bombs, so they decided to attack the marathon.
Earlier Thursday, the body of Tamerlan Tsarnaev, who was the subject of a massive manhunt and died in a gunbattle with police, was claimed on Thursday.
Department of Public Safety spokesman Terrel Harris said a funeral home retained by Tsarnaev's family picked up the 26-year-old's remains. He had no more information.
The medical examiner determined Tsarnaev's cause of death on Monday, but officials said it wouldn't become public until his remains were released and a death certificate was filed. It was unclear on Thursday evening whether the death certificate had been filed.
Tsarnaev's widow, Katherine Russell, who has been living with her parents in North Kingstown, R.I., learned this week that the medical examiner was ready to release his body and wanted it released to his side of the family, her attorney Amato DeLuca said days ago.
Tsarnaev's uncle Ruslan Tsarni, of Maryland, said Tuesday night the family would take the body.
"Of course, family members will take possession of the body," Tsarni said. "We'll do it. We will do it. A family is a family."
Tsarnaev, who had appeared in surveillance photos wearing a black cap and was identified as Suspect No. 1, died days after the bombing.
The April 15 bombing, near the marathon's finish line, killed three people and injured more than 260 others. Authorities said Tsarnaev and his younger brother later killed a Massachusetts Institute of Technology campus police officer and carjacked a driver, who escaped.
Authorities said the Tsarnaev brothers during the gunfight with police set off a pressure cooker bomb and tossed grenades before the older brother ran out of ammunition.
Police said they tackled the older brother and began to handcuff him but had to dive out of the way at the last second when the younger brother, 19-year-old Dzhokhar Tsarnaev, drove a stolen car at them. They said the younger brother then ran over his brother's body as he drove away from the scene to escape.
Dzhokhar Tsarnaev was captured later, wounded and bloody, hiding in a tarp-covered boat in a suburban Boston backyard. He is in a federal prison and faces a charge of using a weapon of mass destruction to kill.
The Tsarnaev brothers' mother insists the allegations against them are lies.
Three of Dzhokhar Tsarnaev's friends, college classmates, were arrested and were accused of helping after the marathon bombing to remove a laptop and backpack from his dormitory room before the FBI searched it.
A top Republican senator on Thursday asked President Barack Obama's administration to explain how one of the students, who's from Kazakhstan, entered the United States without a valid student visa.
Sen. Chuck Grassley, R-Iowa, in a three-page letter to Department of Homeland Security Secretary Janet Napolitano, asked for additional details about the student visa applications for Azamat Tazhayakov and Dias Kadyrbayev, college roommates from Kazakhstan charged with obstruction of justice in the marathon bombing case, and how Tazhayakov was allowed to re-enter the United States in January.
Tazhayakov was a student at the University of Massachusetts Dartmouth when he left the country in December. In early January, his student visa status was terminated because he was academically dismissed by the university.
The third student arrested, Robel Phillipos, was charged with willfully making materially false statements to federal law enforcement officials during a terrorism investigation.
The lawyers for the Kazakh students said their clients had nothing to do with the bombing and were just as shocked by it as everyone else. Phillipos' attorney said the only allegation against him is "he made a misrepresentation."
The Associated Press contributed to this report
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