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Harmony Montgomery died in agony, moaning and gurgling after a beating at the hands of her own father as he chomped down a Burger King meal and shot himself up with opioids, according to prosecutors who delivered their opening statements in her father's murder trial Thursday morning.

Adam Montgomery, a 33-year-old violent New Hamsphire felon and drug addict, had somehow been awarded custody of the 5-year-old girl after his release from prison, even as her brother's adoptive parents were actively trying to add her to their family.

His fate is in the hands of a Manchester jury after lawyers made their closing arguments in his two-week-long trial Wednesday. 

While collecting welfare on her behalf, he lived out of a car with his now-estranged wife Kayla, whom he is also accused of beating, and continued to use drugs, prosecutors said.

FATHER OF SLAIN NEW HAMPSHIRE GIRL ORCHESTRATED SAVAGE PLOT TO HIDE HEINOUS CRIME: PROSECUTORS

Adam Montgomery enters the courtroom for jury selection

Adam Montgomery enters the courtroom for jury selection at Hillsborough County Superior Court in Manchester, New Hampshire, on Tuesday, Feb. 6, 2024. Montgomery is accused of killing his 5-year-old daughter, Harmony. (David Lane/Union Leader via AP, Pool)

"Kayla was terrified to see what he had done," Assistant New Hampshire Attorney General Christopher Knowles told the court. "She could hear the moaning – sounds that eventually stopped."

On Dec. 7, 2019, the day investigators believe Harmony died, she had soiled herself in her sleep, prosecutors said. Her father allegedly beat her over the head viciously, telling his wife, "I think I really hurt her this time."

Missing-Girl New-Hamsphire

Assistant New Hampshire Attorneys General Christopher Knowles, left, and Benjamin Agati stand as potential jurors enter the courtroom at the Adam Montgomery murder trial in Manchester, New Hampshire, on Tuesday, Feb. 6, 2024. (David Lane/Union Leader via AP, Pool)

Then they went to a methadone clinic, picked up drugs, and got fast food, prosecutors said in their opening statement.

Little Harmony had another accident, prosecutors said. Her father turned around and allegedly began pummeling her in the head and face.

Crystal Sorey on witness stand

Crystal Sorey, Harmony Montgomery's biological mother, testifies at the murder trial of the little girl's father, who is accused of brutally beating the child to death. (WFXT)

HARMONY MONTGOMERY CASE: MISSING NEW HAMPSHIRE GIRL'S FATHER ADAM INDICTED ON 2ND-DEGREE MURDER CHARGE

Harmony started moaning and gurgling, according to prosecutors. Her dad ate his food, then shot up some drugs.

When he realized his daughter was dead, he allegedly stuffed her into a duffle bag. Prosecutors say he transferred her body multiple times and kept it for weeks before chopping up her remains and disposing of them somewhere outside Boston.

Montgomery opted out of attending the first day of his murder trial after showing up in court smiling and sticking his tongue out at photographers during jury selection earlier this week.

Kevin Montgomery on witness stand

Kevin Montgomery, the uncle of Adam Montgomery, testifies during the trial. He is one of the first family members to report allegations of physical abuse to law enforcement in connection with his nephew's treatment of murdered 5-year-old Harmony Montgomery. (WFXT)

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Kayla Montgomery is also facing charges in connection with the girl's disappearance, including allegedly lying to a grand jury and collecting welfare payments on the missing girl's behalf for months after her death.

Her husband's defense lawyers tried to blame her for Harmony's death, accusing her of lying to prosecutors about Harmony's final moments.

Undated portraits show Harmony Montgomery smiling

Harmony Montgomery was reported missing two years after she was last seen. Now, her father Adam is on trial for her murder. (National Center for Missing and Exploited Children)

As part of a plea deal, Kayla Montgomery gave investigators damning testimony implicating her husband on charges that include second-degree murder, abusing a corpse and falsifying evidence. She told detectives that Adam Montgomery kept Harmony's corpse for three months, repeatedly hiding it in new places before allegedly dumping it somewhere near Boston. Although the girl's body has not been recovered, investigators allegedly found her DNA in at least one location where he allegedly stowed it.

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Manchester police declared her a missing person in December 2022 after they said they discovered the girl had been unaccounted for since late 2019. 

Adam Montgomery enters the courtroom for jury selection ahead of his murder trial

Adam Montgomery didn't attend the first day of his murder trial after showing up in court smiling and sticking his tongue out at photographers during jury selection earlier this week. (David Lane/Union Leader via AP, Pool)

Police are still searching for her remains, and anyone with information is asked to call the dedicated tip line at 603-203-6060.

Her noncustodial biological mother, Crystal Sorey, repeatedly pressed authorities for information on her daughter's whereabouts after Adam Montgomery refused to let her speak to her. Ultimately, she discovered the child had never even been enrolled in school.

The case prompted an investigation into Massachusetts' handling of the child custody case that placed Harmony out of foster care and back with her father – a man who shot a drug dealer during a robbery and spent years in prison.

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He is already serving a minimum of 32 and a half years in prison for an unrelated firearms case and has pleaded guilty to lesser charges connected to her death. At his sentencing in that trial, he denied killing Harmony.

"I could have had a meaningful life, but I blew that opportunity through drugs," he added. "I loved my daughter unconditionally, and I did not kill her."