BOSTON -- A Massachusetts man killed his girlfriend's young daughter, dumped the child's body and threatened to kill the woman if she told anyone about the death, prosecutors alleged during opening statements in his murder trial Tuesday.

"If you tell anyone, I will kill you … She just died. It was her time. She was a demon," prosecutors say Michael McCarthy told Rachelle Bond.

McCarthy is charged with first-degree murder in the death of Bond's daughter, 2-year-old Bella Bond, whose body was found washed up on a Boston Harbor island inside a trash bag in 2015. The girl was known only as Baby Doe for months as authorities struggled to identify her, and a composite image of the girl was shared millions of times on social media in a massive publicity campaign.

McCarthy was living with Rachelle Bond at the time of the girl's death. Prosecutors said Rachelle Bond was "fascinated by his interest in the supernatural" and that McCarthy sensed "evil spirits" in the apartment. The relationship, prosecutors said, quickly spiraled into a "cycle of increasing heroin abuse."

Bella Bond, left, and a composite sketch released of the child before she was identified CBS Boston/Massachusetts State Police

Prosecutor David Deakin told jurors that Rachelle Bond told authorities she saw McCarthy in June 2015 beating her daughter in the stomach, after which the child wasn't moving or breathing. Some time later, Deakin said, McCarthy told Rachelle Bond to get in the car, and drove to South Boston, where he threw a duffel bag containing Bella's body into the water.

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Prosecutors say divers later found a weighted bag in the water.

Authorities only identified the girl when Rachelle Bond texted a friend of McCarthy's three months later, telling him what had happened. She didn't initially report the death because she was afraid of McCarthy, Deakin said. The friend, who went to police, is expected to testify. Rachelle Bond has agreed to testify for the prosecution and has pleaded guilty to helping dispose of the child's body.

Deakin said McCarthy lied to state police when he was interviewed, saying he didn't know the girl was dead, and didn't show concern for the child.

"He knew the truth – that he had killed Bella Bond and dumped her body – would not set him free," Deakin said.

Photos released by the Suffolk County District Attorney's Office on Sept. 18, 2015 show Rachelle Bond (left), mother of 2-1/2-year-old Bella Bond, and Michael Patrick McCarthy. Reuters/Suffolk County District Attorney's Office

Defense attorneys argue that McCarthy was shocked by news of the girl's death and placed the blame on the girl's mother, who they portrayed as a drug addict who was "paranoid, delusional and not living in the real world." Rachelle Bond had two older children who were removed from her custody over neglect, defense attorney Jonathan Shapiro said.

It was in fact Rachelle Bond who was obsessed with demonology, and believed a "demon was trying to get into Bella, as a ploy to get into Rachelle," Shapiro said.

"She was projecting her own delusions onto Michael McCarthy," Shapiro said. "She blamed him for what she in fact did."

Shapiro called the prosecution's alleged motive "absurd" and said there's "nothing that will support the claim that Michael was obsessed with demons. He lived in the real world."

McCarthy has denied killing the girl.

Bella Bond's biological father, Joe Amoroso, was present in the courtroom Tuesday morning, reports CBS Boston. According to prosecutors, Amoroso and Rachelle Bond met at the Occupy Boston camp in 2011.

Amoroso claims to have never seen his daughter while she was alive. Prosecutors said that when he came to Boston expecting to see his daughter, Rachelle told him she wasn't there, and that he assumed the state had taken her.