The official said the suspects, Tamerlan and Dzhokhar Tsarnaev, advanced the date of their attack because they completed building bombs more quickly then they originally anticipated.
The official declined to be identified and did not offer more details. Police say the brothers detonated two bombs made with pressure-cookers in the April 15 attack on the Boston Marathon that killed three people and wounded 264.
An attack on Boston's packed July 4 celebrations would have carried the extra symbolism of disrupting the city's widely followed Independence Day celebrations.
Citing unnamed officials, The Boston Globe reported on its website that Dzhokhar Tsarnaev, the younger brother who was captured by police four days after the bombing, told investigators the pair discussed detonating their explosives at the city's famed celebration on its Charles River Esplanade.
News of the alleged July 4 attack plan and other details supplied by Tsarnaev to investigators was earlier reported by The New York Times and other media outlets.
NBC News, also citing unnamed officials, reported Tsarnaev told investigators the bombs were made in the home of his brother Tamerlan, who was killed in a shootout with police early on April 19.
The Times reported the ethnic Chechen brothers also considered suicide attacks and that they had viewed online sermons by Anwar al-Awlaki, a radical US-born cleric who was killed by an American drone strike in Yemen in 2011.
There is no indication the brothers communicated with Awlaki, however, the newspaper reported on its website. What, if any, ties the two suspects had with foreign militants is a key question for investigators trying to determine how the pair became radicalized.
How they selected their target would also shed light on their mindset. Mitch Silber, executive managing director at K2 Intelligence and former head of intelligence analysis at the New York City Police Department, said a July 4 attack in Boston might have been more deadly given the fact that greater numbers of people gather for the city's annual celebration.
Former federal prosecutor Mark Rasch said 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." Tamerlan Tsarnaev was 26 when he was killed in the shootout with police in Watertown, Massachusetts.
Dzhokhar Tsarnaev, 19, was wounded in the shootout and captured later that day. Both are also suspected of killing a university police officer.
Another officer was badly wounded in the Watertown confrontation. Dzhokhar Tsarnaev has been charged with crimes in connection with the bombing that could carry the death penalty if he is convicted, and is being held at a prison medical facility in Devens, Massachusetts.
Suspects body claimed
Tamerlan Tsarnaev's remains were claimed on behalf of his family on Thursday. His body had been kept at a Boston facility for more than a week.
Terrel Harris, a spokesman for the Office of the Chief Medical Examiner of Massachusetts, said a funeral services company retained by the family had claimed the body. Harris declined to provide details including the cause of death or where the body had been taken.
On Tuesday, Tsarnaev's widow, Katherine Russell, said through an attorney that she wished his remains to be released to the Tsarnaev family. Russell's attorney could not immediately be reached on Thursday.
Investigators have questioned Russell as they seek clues about how the suspects allegedly built the two bombs used in the attack and whether they had help. The Tsarnaevs' parents previously lived in Cambridge, Massachusetts, but have since returned to Russia.
Other relatives remain in the United States, including an uncle, Ruslan Tsarni of Montgomery Village, Maryland.
Officials said on Thursday that three men who had been charged with interfering with the investigation of the bombing were in custody at a jail in Middleton, Massachusetts, a small town about 20 miles (30 km) North of Boston.
The three 19-year-olds - Azamat Tazhayakov, Dias Kadyrbayev and Robel Phillipos - had been transported to the Essex County Correctional Facility in Middleton on Wednesday after they were charged in Boston.
Authorities have described them as college friends of Dzhokhar Tsarnaev.