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
A newborn baby girl has been killed by dogs after they attacked her while she was lying on a sofa.
It is plausible to say we haven't had our last vote or say on the nature of Britain's relationship with the EU.
After Roy Hodgson's resignation following England's humiliating exit from Euro 2016, who are the leading contenders to replace him?