A six-month investigation into the alleged sexual assault of a seven-year-old girl is being called "one of the most heinous crimes" Hamilton police officers have had to deal with.

The investigation left nine people in handcuffs, including the child's mother and her boyfriend, as police said they uncovered a "horrific" sexual assault where the girl was allegedly being offered up "to be sexually assaulted" in an online advertisement.

Det. Sgt. David Dunbar with the Hamilton police victims of crime branch said the investigation that began last May after the force received a tip from the Catholic Children's Aid Society in Hamilton has left many officers struggling.

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"It's a very unique case because we're not used to a victimization of this nature — it's really graphic," Dunbar told The Canadian Press.

"It's one of the worst, most heinous crimes we've seen because we've got multiple offenders and one young victim."

The investigation, dubbed Project Links, started after the tip when officers interviewed the girl and she told them she was being sexually assaulted by her mother's boyfriend, he said.

Officers seized numerous electronic devices during two raids arrants that contained many "horrific, awful" images of the girl, Dunbar alleged.

"We uncovered a young female victim who was sexually assaulted by more than one person and made available to be sexually assaulted through Craigslist," Dunbar said at a news conference earlier Thursday.

Following the interview with the girl, Dunbar said a 34-year-old man, who police did not identify to protect the identity of the child, was arrested on May 3 and faces 40 charges that include sexual assault, sexual interference, making and possessing child pornography.

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As part of those charges, police allege the man also sexually assaulted the seven-year-old girl's 16-year-old sister.

The girl's mother — a 39-year-old woman from Hamilton — was arrested in September and charged with failing to provide the necessaries of life to a child. She was released on a promise to appear.

The officers continued to pull the string on the case, Dunbar said, as they had to analyze the images and where they were being sent.

"To hear it is one thing, but to go through it and have to visualize and deal with images and graphic sexual violence against children is very difficult, very difficult," he said.

Dunbar said four other people were arrested, including two men from Hamilton aged 36 and 50, and two people from Waterloo, Ont. — a 38-year-old man and a 48-year-old woman.

Dunbar said investigators involved in Project Links became worried about children in other areas, so they shared information with other forces as well as U.S. Homeland Security because Craigslist is based in the United States.

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That led to the arrest of three other men. In early September, Dunbar said, Niagara regional police arrested a 52-year-old St. Catharines, Ont., man on charges of making arrangements to commit sexual interference and possession of child pornography.

That force also arrested Geoffrey Burnet, 48, of Kitchener, Ont., for allegedly making and distributing child pornography. Dunbar said the Niagara force shared information about Burnet that led Hamilton detectives to review a historic sexual assault involving another girl — who is now an adult — that was unrelated to the current investigation.

Burnet was charged in late September with sexual assault, sexual interference and invitation to sexual touching.

Dunbar said a second adult survivor of childhood sexual abuse came forward and Burnet was charged with two counts of indecent acts.

The detective said Burnet is a former elementary school teacher in several boards in southern Ontario.

In early November, Hamilton police shared information from the expanding investigation with Chatham-Kent and Waterloo regional police forces that led to the arrest of a 35-year-old Waterloo man on charges of making and possessing child pornography.

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As difficult as the case is, there is a bright spot, Dunbar said.

"The professional and personal commitment that has gone into this case in seeking out all those responsible for hurting that little girl is phenomenal," Dunbar said.

And both girls are doing well now. They're in a safe spot."

Dunbar said eight of the people charged were not named because a publication ban was ordered by the court to protect the identity of the girl "and to ensure the integrity of the ongoing investigation."

Police believe there are other alleged victims.

"We continue to follow the evidence — there is a lot more information to investigate," Dunbar said.