Bernalillo County sheriff's spokesman Aaron Williamson has said that the motive of the teenage boy who shot dead five people at a home in New Mexico is not yet known, the New York Post reports.
Mr Williamson also said that investigators were trying to determine any connection of the teenager to the victims and if the victims were related.
More top news
Mr Kim has made two recent visits to China.
The study looked at the development of 422 children over the course of eight years.
The president said they will come into force if China refuses to change its practices.