BOSTON — A mother was arrested and her boyfriend was 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 and who became known as Baby Doe, a prosecutor said Friday after a month long campaign to learn the girl’s identity.

Suffolk District Attorney Daniel Conley said he authorized the murder charge for the boyfriend, Michael McCarthy, and the mother, Rachelle Bond, was under arrest for accessory to murder after the fact.

The girl became known as Baby Doe as investigators trying to find out who she was and how she died launched a massive social media campaign including a composite sketch of her, showing her chubby cheeks and brown eyes. Her real name was Bella Bond, prosecutors said Friday.

“At just shy of 3 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.

After Baby Doe’s body was found in June, police immediately appealed to the public for help. The National Center for Missing and Exploited Children, using photos of her remains, created a composite image of what she might have looked like when she was alive.

Photos of Bella posted on her mother’s Facebook page looked strikingly similar to the composite image.

Conley didn’t 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?”

State House Speaker Robert DeLeo said Bond and McCarthy were blaming each other for the death.

McCarthy, 35, was being treated at a hospital for a medical condition unrelated to the case, prosecutors said. Bond, 40, was in custody.

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