The Boston bombers had planned their attack for July 4, the US Independence Day, but changed their plans as the explosives were ready much before, according to investigators.
And, in fact, they had no specific plans for where to plant them in April. They drove around the city before deciding on the marathon finish line, the officials said.
The pressure cooker bombs, which killed three people and injured more than 260, were made by Dzhokar Tsarnaev in brother Tamerlan Tsarnaev's apartment in Cambridge, Massachusetts, with the help of the al Qaeda magazine Inspire.
While Tamerlan, died in a firefight few days later, his brother is in prison. He spoke to the FBI investigators at the hospital where he was taken after capture. He has since clammed up.
Tsarnaev also told the investigators that he and Tamerlan had watched videos of Anwar al-Awlaki, the American-born cleric killed by a drone in Yemen.
But investigators haven't found any evidence if the brothers were in touch with al-Awlaki.
Tamerlan may have been in touch, however, with a known militant Mahmoud Mansur Nidal during his six-month say in Dagestan, Russia in 2012, according to US and Russian media reports.
Dzhokhar also told investigators, as reported earlier, that the brothers were angered by US wars in Iraq and Afghanistan and believed there was a conspiracy against Muslims.
But they had acted on their own, without help from outside.
Investigators, however, are probing into the possible role of Tamerlan's widow Katherine Russell either in preparing for the attacks or sheltering the brothers after.