The sources said the surviving suspect, Dzhokhar Tsarnaev, told authorities after his capture that he and his older brother, Tamerlan, only changed their plan and hit the marathon because they were surprised at how quickly they were able to build functioning explosives.
Dzhokhar also reportedly told the FBI that the bombs were constructed in Tamerlan's Cambridge apartment.
In the end, the Tsarnaev brothers stand accused of setting off a pair of bombs near the finish line of the Boston Marathon on April 15, killing three people, including an 8-year-old boy, and injuring more than 260 others. Tamerlan was killed in a shootout with police days later, while Dzhokhar was injured and later captured.
Previously, Dzhokhar reportedly said the brothers had learned how to build bombs online and were not funded or directed by any foreign government or rogue group. Still, authorities are investigating a 2012 trip Tamerlan took to Russia where he met with known Islamist militants in Dagestan.