Two people were arrested overnight as a result of police inquiries, which were said to be at a very early stage.
Detective Superintendent Mark Ridley, who is leading the investigation for West Yorkshire Police, said: "This is a tragedy for the family and they are absolutely devastated. Specially trained officers are currently working with them.
"I am appealing directly to anyone who saw the vehicle being driven or the incident to get in contact.
"I need the community to come forward and tell me what has happened and what they have seen to help bring justice to the family."
Any witnesses are asked to contact police on 101 or Crimestoppers on 0800 555111.
More top news
Clare Waight Keller said it was an "enormous honour" to have been given one of fashion's most coveted jobs ahead of the royal wedding.
As temperatures rise over the coming few days, the risk of thunderstorms across some southern parts
Margot James was asked about her own experiences of receiving abuse on the internet.