(CNN) -- Investigators have found explosives residue in the apartment that bombing suspect Tamerlan Tsarnaev shared with his wife and young daughter, a source briefed on the investigation said Friday.
The residue turned up in at least three places at the apartment in Cambridge, Massachusetts, the source said: the kitchen table, the kitchen sink and the bathtub.
U.S. law enforcement officials briefed on the progress of the investigation have provided CNN with other critical details:
-- Tamerlan's younger brother, Dzhokhar, told investigators that he and his brother built the devices in Tamerlan's home;
-- He also said the brothers originally planned a suicide attack on the city's massive Independence Day celebration, which draws about 500,000 people and is televised nationally on CBS;
-- Dzhokhar Tsarnaev also told investigators that he and Tamerlan chose to target the Boston Marathon only a day or two before the event. They changed their plans because their bombs were ready sooner than they expected.
The officials spoke on background because they were not authorized to speak with the media.
Meanwhile, investigators searched areas in and around Dartmouth, Massachusetts, on Friday, the FBI said. They were following up on tips about loud explosions heard in the area over the past several months to see whether they were tests conducted by the suspects, one official told CNN. He said nothing was found.
Fashioned out of pressure cookers that detonated near the finish line on April 15, the bombs killed three people and wounded more than 260 others. Authorities say they believe the brothers later killed a Massachusetts Institute of Technology police officer.
Tamerlan Tsarnaev died in the early hours of April 19 following a firefight with police. Dzhokhar eluded capture until later that day, when he was found hiding under a boat tarp in the backyard of a Watertown, Massachusetts, home.
He has been charged with using a weapon of mass destruction and faces the death penalty if convicted.
Tamerlan Tsarnaev's body is now at the Graham Putnam & Mahoney Funeral Parlors in Worcester, Massachusetts, and services are being planned, the home's director said Friday. The mayor's office said Tsarnaev's funeral service will be held in Worcester, but the exact location was unclear.
Tsarnaev's uncle Ruslan Tsarni -- who has said his nephews had brought shame on the family and all Chechens -- claimed his body, according to a family spokeswoman.
The family plans an independent autopsy before burying the body in a Massachusetts cemetery, spokeswoman Heda Saratova said.
Katherine Russell, Tsarnaev's widow, has remained largely out of view since her husband's death, staying inside her parents' Rhode Island home.
Her attorney, Amato DeLuca, says the 24-year-old knew nothing about plans to bomb the race, and reports of her husband's involvement came as an "absolute shock" to Russell and her family.
Dzhokhar Tsarnaev, who suffered gunshot wounds to the head, neck, legs and hands, is being held at a federal Bureau of Prisons medical facility in Devens, Massachusetts.
Three of his friends have also been charged in connection with the bombing.
Azamat Tazhayakov and Dias Kadyrbayev were charged Wednesday with conspiring to discard potentially incriminating items from Dzhokhar Tsarnaev's dorm room. Robel Phillipos was charged with making false statements to investigators.
The FBI is examining Dzhokhar Tsarnaev's laptop, two federal law enforcement officials told CNN.
Authorities have said they believe the brothers acted alone, but are investigating whether they could have learned from or been aided by terror groups, including groups overseas.
Of particular interest has been Tamerlan Tsarnaev's 2012 trip to the semiautonomous Russian republic of Dagestan, home to numerous Islamic militant groups that have warred against Moscow's rule.
Russian authorities asked U.S. officials to investigate Tamerlan Tsarnaev some months before the trip, saying they believed he was becoming increasingly involved with radical Islam. The FBI investigated but found no evidence of extremist activity and closed the case.
U.S. officials learned after the bombings that Russian officials had intercepted a 2011 phone call between the suspect's mother, living in Dagestan, and one of her sons in which they reportedly had a vague conversation about jihad.