BOSTON » A mother has been arrested and her boyfriend is charged with murder in the death of a little girl whose body was found inside a trash bag on a Boston Harbor island this summer, a prosecutor said Friday, after a monthslong campaign to learn the child’s identity.

Suffolk District Attorney Daniel Conley said he authorized a murder charge for Michael McCarthy, 35, and the girl’s mother, Rachelle Bond, 40, is under arrest for accessory to murder after the fact.

“At just shy of three years old, Bella Bond was a true innocent,” Conley said at a news conference announcing the charges.

“This child, whose very name means beauty, was murdered,” he said.

The girl was known as Baby Doe as investigators launched a massive social media campaign to find out who she was and how she died.

Police immediately appealed to the public for help. Using photos of her remains, the National Center for Missing and Exploited Children created a composite image of what the girl might have looked like when she was alive. Within two weeks, the image of the chubby-cheeked, brown-eyed girl had reached an estimated 47 million people on Facebook.

Photos of Bella posted on her mother’s Facebook page made it clear that she looked strikingly similar to the composite image.

House Speaker Robert DeLeo, D-Winthrop, said Bond and McCarthy are blaming each other for the girl’s death.

“It appears as though it was a situation of the boyfriend who was involved and apparently, I think, mother and boyfriend sort of blaming each other in terms of who harmed the child.”

Conley said McCarthy was being treated at a Boston hospital for a medical condition unrelated to the case.

Both Bond and McCarthy are due to be arraigned on the charges Monday.

Officials from the state’s child protection agency said they had contact with Bella twice, for four months in 2012 and three months in 2013. In both cases, officials described the involvement as “support for neglect,” and the cases were later closed.

Between 2001 and 2006, the Department of Children and Families terminated parental rights for two of her mother’s older children. One of the children was later adopted by her maternal grandmother; the other was adopted by someone outside the family, DCF officials said.

Rhonda Mann, a spokeswoman for the Executive Office of Health and Human Services, said the agency has been working with law enforcement to help police identify the girl.

“Now that we know her name, the story is no less tragic. DCF has not had an open case with this family for over two years, but did have brief involvement with Baby Bella as an infant,” she said in a statement.

The girl’s body was found June 25 inside a trash bag on Deer Island in Winthrop by a woman walking a dog.

Despite the widespread publicity, a text tip line and billboards with the girl’s image, investigators were frustrated for months as they tried to learn her identity. They got their big break after Boston police received a tip and a search warrant was executed Thursday at Rachelle Bond’s apartment.

Conley would not say specifically how her body got to Deer Island, but implied that her mother placed her there. He said evidence investigators uncovered led them to charge her with being an accessory after the fact to murder.

Bond had a history of drug and prostitution arrests. In 2006, she was arrested after a car she was a passenger in was stopped for a traffic violation. When Bond got out of the car, police noticed she was sitting on a syringe. She was charged with possession of a hypodermic syringe.

In 2008, she was charged in a sting operation with offering to perform sexual acts for a fee on a male police officer posing as a john.

On Rachelle Bond’s Facebook page, she appears to be a doting mother, posting numerous photos of her daughter, who she calls “Bell” and “monkey.”

In a posting on Aug. 18, 2014, Bond wrote: “My Bella loved her birthday party and all the awesome things she got. Broke the bank on it but it’s all worth it to see her happy and laughing and smiling. Love it.”

A few weeks later, on Sept. 15, 2014, a photo shows Bella wearing a tight-fitting, one-piece romper. “Look at my Bell,” Bond wrote. “The outfit isn’t supposed to be a catsuit …Love her to death. I am on top of the world because of her. My life is complete again and worth living. I give her the world and more if I can…”

Conley said he received at least 10 offers from people who wanted to pay for the little girl’s burial, including Cardinal Sean O’Malley.

Conley would not comment on how the girl died, but said more information may be released when McCarthy and Bond are arraigned. There were no obvious signs of trauma to her body.

Police chased down tips from around the world, but experts determined pollen on the girl’s blanket and leggings and in her hair came from trees found in New England.

Conley did not comment on a possible motive.

“That’s always a perplexing matter for us,” he said. “Why does anyone kill a 2-year-old child? What could she have done?”

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Associated Press writers Collin Binkley, Bob Salsberg and Steve LeBlanc contributed to this report.