Attending the memorial service, Chief Constable Sue Sim said of Pc Rathband: "He was gunned down in tragic circumstances doing the job of constable he loved.
"My heart goes out to Kath and the children and the rest of the family."
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Some 66 people have been killed after the plane they were travelling on crashed in southern Iran.
The letter calls for an end to harassment, abuse, and impunity in a world "ripe for change".
Police say that anyone who sees Whitehouse should not approach him, but instead call them on 999 and quote incident 23 of 18 February.