People have taken to Facebook to post messages on a page set up to pay tributes to Sarah Groves.
Underneath a photograph of her smiling, Charlene Carter said: "That beautiful smile I will never forget."
Donna Stacey said: "Waste of a beautiful life just goes to show the world we live in."
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He has asked his followers for help in tracking the man down.
The president said it was a “very sad day for journalism”.
A summit has been pencilled in for February following last year’s icebreaker in Singapore.