Tsarnaevs initially planned to target Boston on July 4
Three people were killed while over 260 over injured in the attacks.
Dzokhar, 19, said they had later decided to launch the attack earlier because they contemplated making the pressure-cooker bombs faster than they expected, according to officials, a report published in The Washington Post said.
Dzokhar said after making the bombs in his brother's apartment in Cambridge, Massachusetts, they drove around Boston in a car and considered several targets. He said they finally decided to explode the devices near the finish line of the historic marathon on Patriot's Day, a major holiday here.
The two brothers had engaged the police in a massive manhunt on the night of April 19 after shooting a police officer dead at the Massachusetts Institute of Technology. Dzokhar was later found in a seriously injured condition from underneath a boat cover at Watertown.
The Tsarnaev brothers, who originate from Chechnya in Russia, reportedly carried out the attacks for they were angered by the US wars in Afghanistan and Iraq in the last decade although they were not involved with any extremist group. They wanted to defend Islam, Dzokhar told the interrogators later.
Meanwhile, the body of Tamerlan was claimed by a funeral home retained by his family on Thursday, medical sources said.