Police want to speak to a witness who helped an elderly man at the Household Waste and Recycling Centre in Birstall before he later died in hospital.
The man officers want to trace helped the 82-year-old dispose of garden waste from his Skoda vehicle at the Nab Lane facility on Thursday April 21.
It is thought he witnessed the man suffer a serious head injury.
The pensioner is thought to have either fallen or collapsed and was taken to hospital but died later that day.
Officers are hoping to speak to the man as a witness to the incident.
The witness is described as white, wearing shorts and possibly aged in his 40s.
Anyone with any information is asked to contact DC Gemma Smith at Kirklees CID via 101 quoting reference number 13160180979B.