A family liaison officer has read out a statement on behalf of Craig De la Haye - a Jersey man whose wife Marta and five-year-old daughter Julia were both killed by Damian Rzeszowski.
They were both visiting the property where the Rzeszowskis lived when Damian turned on his own family.
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Temperatures will remain close to freezing.
The prime minister's announcement came as international leaders gather in Paris on the second anniversary of the climate change plan.