One source said the Tsarnaev brothers' original intent when they hijacked a car and its driver in Boston last Thursday night was to drive to New York with seven bombs to set them off.
But the abducted driver escaped when the car pulled into a gas station and both brothers got out, one to pump the gas and the other to pay for it at the station's night cashier, the source said, based on what Dzhokhar told investigators in hospital, where he is recovering from gunshot wounds.
The driver then called police and a shootout with the bombing suspects began, eventually leading to the killing of older brother Tamerlan, 26, and the arrest of Dzhokhar, 19. The pair threw explosive devices out of the car window in the confrontation, police said.
Two other sources could not confirm that the suspects were headed to New York last Thursday night but said that law enforcement authorities believe the ethnic Chechen brothers had planned to travel to the city to detonate bombs at some point.
Police say the pair planted and detonated two pressure-cooker bombs near the finish line of the Boston Marathon on April 15, killing three people and injuring 264. The brothers were identified by the FBI as suspects from pictures and video at the scene.
Prosecutors formally charged Dzhokhar Tsarnaev with the bombings in a hearing before a U.S. magistrate judge in his hospital room on Monday, accusing him of crimes that carry the possibility of the death penalty.
Tsarnaev has spoken to FBI investigators. The first law enforcement source said that he initially told questioners that he and his brother had planned to go to New York "to party" and then gave details about possibly setting off bombs in the city.