The official said Dzhokhar Tsarnaev, 19, told investigators the bombs were ready earlier than he and his brother, Tamerlan Tsarnaev, had expected, so they moved up the date and the April 15 marathon was selected as the target only a day or two in advance, CNN reported Friday.
The July 4 target was not specified.
Citing a source it said had been briefed on the investigation, CNN reported Friday investigators have found explosive residue at the Cambridge, Mass., apartment Tamerlan Tsarnaev, 26, shared with his wife, Katherine Russell, and their young daughter. The source said residue samples from the kitchen table, kitchen sink and bathtub tested positive.
The bombs used at the marathon blasts were constructed in the home, a U.S. official said.
Russell has been living at her parents' home in North Kingstown, Mass., since her husband died in a shootout with police April 19. The federal official said it was unclear what, if anything, Russell may have known or suspected.
Federal investigators were trying to determine whether Russell, 24, knew anything about the bomb plot, The Washington Post reported Friday.
Citing officials it did not name, the Post said investigators found radical Islamist content on her computer, including al-Qaida's Inspire magazine. It was unclear who downloaded the content, Russell, her husband or a third party.
Her attorney, Amato DeLuca, has said she was not involved in the plot and knew nothing of her husband and brother-in-law having participated in it.
The newspaper said federal, state and local authorities were searching a wooded area near Dartmouth, for evidence the Tsarnaev brothers tested explosives. People living nearby had reported hearing loud noises in the area two weeks before the Boston Marathon bombing.
In addition to the three people killed, more than 260 were wounded in two bomb explosions at separate locations near the finish line of the marathon.
The revelation about the July 4 plan came Thursday, the same day the body of Tamerlan Tsarnaev was claimed by his family and picked up by a funeral home, Terrel Harris, a spokesman for the Massachusetts office of the chief medical examiner, said.
Tsarnaev's cause of death won't be released until the death certificate is filed with the Boston city clerk, Harris said.
Tsarnaev's parents in Dagestan won't bring his body back to Russia and they won't bury him until an "independent" autopsy is performed, a family spokeswoman said Friday.
The body is to be interred "somewhere in Boston," the spokeswoman said.
Federal officials said they received a laptop belonging to Dzhokhar Tsarnaev from one of the suspect's friends who has been accused of obstructing justice, CNN reported.
Four people have been charged, including Dzhokhar Tsarnaev, who is accused of carrying out the attack along with his brother. Among the charges Dzhokhar Tsarnaev faces is using a weapon of mass destruction. He sustained injuries when he was captured by police in Watertown, Mass., April 19.
Azamat Tazhayakov and Dias Kadyrbayev -- the friend who turned over the computer to authorities -- were charged Wednesday with conspiring to discard potentially incriminating items from Dzhokhar's dorm room. Robel Phillipos was charged with making false statements to investigators.
The Kazakhstan Foreign Ministry said Thursday it offered consular services to Kadyrbayev and Tazhayakov, who are citizens of that country in the United States on student visas.
"We would like to emphasize that our citizens did not receive charges of involvement in the organization of the Boston Marathon bombings," the ministry said in a statement. "They were charged with destroying evidence."