The crash happened between Staincross and Newmillerdam, with the road shut between Shaw Lane and Warren Lane.
The road is expected to be closed for some time as the investigation continues, officers said, with any witnesses asked to come forward.
Supt Alan Travis, from West Yorkshire Police, said: “This is an absolutely tragic incident which has resulted in the loss of six lives.
“Our thoughts go out to the families of those who have died who we are working with to provide support at this time.”
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