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Some attempts had been made to narrow it down by the time Det Insp Hannah Friend took on the case in 2021.

"We thought maybe this is somebody who slipped through the net and as it turned out it wasn't at all," Det Insp Friend said.

A few months before Joanne Sharkey was arrested, Det Insp Friend got an email from that scientist with the subject heading: "Are you sitting down?"

For Det Insp Friend and her team, that promise made by Callum's graveside in 2021 had been fulfilled.

After two years of ruling out potential suspects from the original list, Det Insp Friend made the decision in 2023 to run a new trawl of the national DNA database.