They decided to launch the attack earlier than planned after completing the homemade pressure cooker bombs ahead of schedule, U.S. media reported.
Tamerlan Tsarnaev, 26, died following a shootout with police four days after the April 15 bombings. The twin explosions near the finish line of the race killed three people and injured more than 260.
- See a timeline of key events in the Boston Marathon bombings case
Dzhokhar Tsarnaev, 19, was arrested on April 19 following a massive manhunt that virtually shut down the city of Boston and several surrounding communities. He faces a charge of using a weapon of mass destruction to kill and a possible death penalty.
Tsarnaev, who is being held in a prison medical facility, told investigators the pair built the pressure cooker bombs at his brother's house and chose the finish line of the race after driving around Boston, according to the New York Times.
The 19-year-old also said he and his brother had watched internet sermons of radical American Islamic cleric Anwar al-Awlaki, who was killed in a U.S. airstrike in Yemen in 2011, although there is no indication they communicated with him, the paper reported.
Law enforcement officials expressed a degree of skepticism over Tsarnaev's account, however, because it is unlikely the pressure cooker bombs could have been completed as quickly as he said given their complexity, the Washington Post reported. Cause of death not released
Tsarnaev provided the details before he was informed of his right to remain silent, U.S. media reported.
The body of Tamerlan Tsarnaev was claimed on Thursday by a funeral home, according to Department of Public Safety spokesman Terrel Harris.
The medical examiner determined Tsarnaev's cause of death Monday, but officials said it wouldn't become public until his remains were released and a death certificate was filed. It was unclear Thursday evening whether the death certificate had been filed.
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.
Three college friends of Dzhokhar Tsarnaev have also been arrested and accused of trying to protect him by going into his dorm room and removing a backpack filled with hollowed-out fireworks three days after the bombing.
The three 19-year-olds have not been accused of any role in the bombings.