But they decided to move up the date as their pressure cooker bombs were ready earlier than they expected, Dzhokhar Tsarnaev, 19, told investigators, CNN reported citing an unnamed law enforcement official.
It is unclear which specific event would have been targeted but one of the biggest July Fourth celebrations in the country is held in Boston, the news channel said.
The annual Boston Pops Fireworks Spectacular hosts 500,000 spectators, according to the event's website. It is televised live nationally on CBS.
CNN also reported that the bombs used in the Boston attack were built in the apartment that the older brother Tamerlan Tsarnaev, 26, who died in a shootout with the police, shared with his wife and child. Tamerlan Tsarnaev's widow Katherine Russell has remained largely out of view inside her parents' North Kingstown home since her husband's death.
Meanwhile, the Federal Bureau of Investigation has got hold of a laptop belonging to Dzhokhar and it may yield some clues into the bombings case, CNN said citing two federal law enforcement officials.
Dias Kadyrbayev, one of Dzhokhar's friends accused of obstructing justice, "turned over the laptop to the FBI," his lawyer told CNN.
The New York Times citing the account that Dzhokhar provided authorities said the brothers picked the finish line of the marathon for the attack after driving around the Boston area looking for alternative sites.
In addition, Dzhokhar told authorities that he and his brother viewed the Internet sermons of Anwar al-Awlaki, a radical American cleric who moved to Yemen and was killed in Sep 2011 by an American drone strike, the Times said.
There is no indication that the brothers communicated with Awlaki before his death.
Tsarnaev made his admission on April 21, two days after he was captured while hiding in a boat in a nearby backyard, to specially trained FBI agents who had been waiting outside his hospital room for him to regain consciousness.
Federal authorities, the Times said are skeptical of Russell's insistence that she played no role in the attack or in helping the brothers elude the authorities after the FBI released photos of them.
The FBI has also decided to send more agents to Russia to assist with the investigation, officials cited by the daily said.
The bureau has been relying on a couple of agents based in the US embassy in Russia to serve as intermediaries with the authorities there, the Times said.
American and Russian investigators in Dagestan, in the turbulent Caucasus region of southern Russia, have been trying to determine what Tamerlan Tsarnaev did during a six-month visit to Dagestan last year, it said.