The person has been charged under juvenile laws with three counts of first-degree murder and one count of falsifying physical evidence in connection with the homicides of a 25-year-old woman and her sons, ages 1 and 4, on Aug. 3. Each died of a single gunshot wound, authorities said.
Law enforcement officers responding to a 911 call found the three bodies inside the residence.
Because the suspect is not an adult, the attorney general’s office said no further information would be released. They didn't share the suspect's name, age or other identifying information, citing legal restrictions on sharing information on children charged with crimes.
Senior Assistant Attorney General Geoffrey Ward didn't say whether the suspect was known to the family or whether searches in various New Hampshire towns over the past week turned up any evidence.
A horrifying discovery
Police found the bodies of 25-year-old Kassandra Sweeney and her two sons, 4-year-old Benjamin Sweeney and 1-year-old Mason Sweeney, in their Wethersfield Drive home on the morning of Aug. 3 while responding to a 911 call. Each died of a single gunshot wound, autopsies found.
Autopsies were conducted last week, and the chief medical examiner determined that all three family members died from single gunshot wounds. Their deaths have been ruled homicides.
Authorities searched for evidence in the case on Saturday in Northfield and Tilton, and again on Wednesday near Interstate 93 in Concord, Canterbury and Tilton, but didn't immediately say if they discovered any evidence related to the case.
As the investigation continued, rumors spread in Northfield, a town of fewer than 5,000 residents north of Concord and southwest of the Lakes Region.
"Every other customer is coming in saying something about the gun and who did it," Barbara Moulton, who works at a local convenience store, told NBC10 Boston.
"The first three or four days people were really scared, like, 'Oh no, who was it?'" she added.
Funeral Saturday for Kassandra Sweeney
A funeral is planned for Kassandra Sweeney and her two sons on Saturday in Concord.
While authorities are looking to bring the family justice, loved ones are seeking closure. Obituaries posted by Bennet Funeral Home in Concord share a glimpse into the young Northfield family.
Benjamin Michael was only 4 years old. He enjoyed learning about dinosaurs, making crafts, and loved movie nights with his parents and little brother Mason.
Mason was only 1. He loved dancing to "Baby Shark." Spending time with his grandparents was a weekend highlight, according to his obituary.
Their 25-year-old mother, Kassandra, made her family the center of her world, spending most of her time building memories with her husband and two kids.
Sean Sweeney speaks
Ward said last week that the attorney general's office had been in contact with the children's father, Sean Sweeney, and he has been "very cooperative and helpful in this investigation." He said the attorney general's victim witness advocate is working with him and his family.
"He is obviously beyond devastated as a result of these crimes," Ward said.
Sweeney spoke about the case for the first time in a Facebook post Sunday morning.
"I wanted to thank everyone for the outreach and support, while things will never be 'normal' again my body finally shut down and let me get some sleep last night and I woke up feeling as 'normal' as possible," he wrote.
Sweeney said he is "unable to confirm anything as to what happened and who is responsible" and called his wife Kassandra "the most amazing, caring, sweetest all around good person that anyone could ever meet," while adding that she loved their children very much.