Boston blast mastermind Tamerlan Tsarnaev had planned to blow up the bomb on the July 4, but rescheduled his plans as his pressure cooker bomb, which he had made in his own apartment, was ready much early.
The crucial information was provided by the lone surviving suspect Dzhokhar Tsarnaev, 19, in his interrogation to the federal investigators, media reports said on Thursday.
Dzhokhar, who was arrested from a Boston neighbourhood on April 18, has been charged with using weapons of mass destruction. If convicted he faces life imprisonment.
Tamerlan, 29, was shot dead in a police exchange of fire on April 18, three days after the Boston Marathon bombing that killed three people and injured more than 200 others.
Body of Tamerlan was claimed and was picked up by a funeral home on Thursday.
The cause of his death has not been released now.
According to Boston Globe, Kheda Saratova, a human-rights worker, said the Tsarnaev family does not intend to bury him until they find an independent coroner to deliver an opinion.
"The family is afraid that if Tamerlan is buried before they get all the answers, many secrets will be buried with him, and this will make it harder for Dzhokhar to defend himself in court," he said.
Citing unnamed officials having firsthand knowledge of the investigation, several media reports said Dzhokhar told investigators that the pressure cooker bomb was made by Tamerlan inside his apartment.
"The explosives used in the Boston Marathon bombings were likely made in the Massachusetts home of suspected bomber Tamerlan Tsarnaev," Fox News reported.
Initially they had planned to attack on July 4 Independence Day celebration on the Charles River Esplanade.
But once they build the bomb faster than they anticipated, they decided to go for it early.
After which they drove around the city to identify the possible locations for the terrorist attack.
"They surveyed these police stations, multiple stations in Boston and one in Cambridge. They built the bombs so fast that they decided to move the whole plan up," an unnamed official was quoted as saying by Boston Globe.
The computer used by Dzhokhar was turned over to the FBI four days after the bomb attack by one of his friend Dias Kadyrbayev, who was arrested by the police on Wednesday.
Photograph: Jessica Rinaldi/Reuters