13 year-old Casey Kearney died on Valentines' Day after she was stabbed in Elmfield Park in Doncaster.
Police want people who were seen near the park just after 1pm that day to contact them.
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The 28-year-old's activities were first logged by Canadian authorities after he shared images via social media.
Katie and Ryan Barrett are trying to raise £100,000 to fund experimental brain tumour treatment for their five-year-old twin son Zack.
Her petition is calling to ensure family courts don't allow known abusers to continue having unsafe access to children.