The adoptive parents of missing New Hampshire girl Harmony Montgomery’s little brother sent a letter to her jailed father Sunday pleading for him to come forward with details that could lead police to her whereabouts.

Harmony has been unaccounted for since late 2019, months after moving in with her father, police said, but they only discovered she was missing in November after her noncustodial biological mother, Crystal Sorey, called police upon learning the 7-year-old had never been enrolled in school in Manchester.

"Not a week goes by that Jamison doesn’t talk about Harmony," Johnathon and Blair Miller wrote to Adam Montgomery, who is being held in a New Hampshire jail in connection with her disappearance. "He loves his sister and wants to have that relationship with her. In fact, he craves it."

Missing Harmony Montgomery and her little brother Jamison at a foster home more than two years ago. 

Missing Harmony Montgomery and her little brother Jamison at a foster home more than two years ago.  (Courtesy: Johnathon Miller)

The children were very close, Blair Miller, who is also a TV journalist for Boston 25, told Fox News Digital Monday.

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He’s only five, and his parents haven’t explained to him the extent of the uncertainty around his sister’s whereabouts, Miller said. On Valentine’s Day last week, Jamison told his parents he wanted to send her chocolates. 

"Maybe someone else is gonna read that letter and think, ‘I do know something, I was there at that house and saw something,’ and maybe that’ll mean something to somebody," Miller said. "So this really wasn’t just a letter to Adam, it’s a letter to help open up any clues or help the investigation."

MISSING HARMONY MONTGOMERY: ADOPTIVE PARENTS' PUSH TO REUNITE SEPARATED SIBLINGS TRIGGERED SUSPICIONS

Manchester police tracked down Montgomery and his wife, Kayla, in December while searching for Harmony’s whereabouts and found the couple living out of a car with no sign of the girl. The discovery kicked off a large-scale investigation involving police in multiple states and federal investigators.

"The U.S. Marshals stopped by our house a couple of weeks ago and brought [Jamison] some patches and a Care Bear, and they just sat with him," Miller said. "They told him, if you need anything from us, give us a call. The problem with that is that he's been trying to use Alexa and call 911 to talk with his friends – but it was very it was such a nice gesture, and they were just trying to show their support for him."

The Millers sent the letter to Montgomery through his attorney. They said Monday afternoon they had not yet received a response.

MISSING HARMONY MONTGOMERY: DAD ACCUSED OF GIVING PARTIALLY BLIND DAUGHTER BLACK EYE IN 2019, OTHER ABUSE

Harmony and Jamison had been in and out of foster homes before the Millers adopted the little boy and Adam Montgomery secured custody of his daughter in a Massachusetts court.

Adam and Kayla Montgomery have both been charged in connection with Harmony’s disappearance. Adam is charged with child abuse and endangering the welfare of a child. Kayla with allegedly collecting food stamps on the girl’s behalf for months after she last saw her.

Montgomery had a history of drug use, as well as prior violent arrests, including for allegedly shooting a man in the face in 2014 during an attempted drug robbery. He later pleaded guilty to lesser charges.

Months after Adam Montgomery gained custody of Harmony in 2019, his uncle Kevin Montgomery reportedly saw the girl with a black eye. Adam Montgomery allegedly told his uncle he had "bashed her around the house."

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Harmony is about 4 feet tall and 50 pounds. She has blonde hair and blue eyes and wears glasses. She’s also blind in her right eye. Manchester police urge anyone with information to call the dedicated tip line at 603-203-6060.

"We still have hope," Miller said. "We see everyone else losing hope, and we still have hope. We have to, for our son. Because that’s what he would expect from us."