In England and Wales and Northern Ireland Parental Child Abduction may be a criminal offence where a child (under 16 years old) is taken out of the UK without either consent of another person who has parental responsibility, or permission from the courts.
In Scotland, abduction may only be considered a criminal offence where the courts have issued an order concerning custody of the child or their removal from the UK, but a parent may still have rights under civil law.
More top news
Political parties across the country suspended their General Election campaigns following the Manchester Arena terror attack on Monday.
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.