The brothers suspected of setting off two bombs at the Boston Marathon originally planned the attack for July 4, law enforcement officials said.
Dzhokhar Tsarnaev, 19, told police he and his brother Tamerlan, 26, planned to set off their bombs at Boston's packed Independence Day celebrations.
Former federal prosecutor Mark Rasch told Reuters that a July 4 attack would have sent a stronger message.
"The essence of terrorism is all about symbolism," Rasch said. "The Boston Marathon just does not have as much of a symbolic feeling as the Fourth of July to the United States."
Instead, the younger Tsarnaev told police that they were able to make the pressure cooker bombs faster than expected and decided to move up the date of their attack.
The Tsarnaev brothers set off two bombs at the finish line of the Boston Marathon on April 15, killing three people and wounding more than 260.
Tamerlan Tsarnaev died in a shootout with police after a days-long manhunt for the brothers. Dzhokhar escaped and was captured that evening after hiding in a boat in a Watertown, Mass., backyard.
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Officials also told NBC that the surviving suspect said the bombs were built at Tamerlan's house, where explosive residue was found.
Tsarnaev told police from his hospital bed that he and his brother were angry about US-led wars in Iraq and Afghanistan. He said they acted alone and were not involved in any extremist or terrorist group.
The Washington Post reports that Tsarnaev told the FBI that he and his brother were partly influenced by online sermons posted by Anwar al-Awlaki, a radical US-born cleric who was killed in a drone strike in Yemen in September 2011.
Dzhokhar Tsarnaev is being held at a prison medical facility at Fort Devens, Mass. He could face the death penalty if convicted.