Triplets from New Zealand are among 13 children killed in a fire at a shopping centre in Qatar.
The two-year-olds were trapped inside the building at a childcare area at the Villagio Mall in Doha.
Nineteen foreign nationals were killed.
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Investigators believe Mr Kushner has significant information relevant to their inquiry, officials told NBC News.
The men were detained as part of an investigation by MI5 and the Metropolitan Police's Counter Terrorism Command.