Mr Piper disappeared from the home of his former partner Hanna Lark in Eatwell Avenue, Paraparaumu, after visiting his three-year-old son on May 15.
His partner returned home after having breakfast to find he had gone, leaving behind his cash card, driver's licence and extra clothing.
A dark green Mazda Marvie 4WD linked to the case had been found at a property at Turakina, south of Wanganui, Detective Senior Sergeant Mike Oxnam said today.
Mr Piper, also known as Son, was seen travelling in the vehicle.
Police also searched the Challenge Kapiti petrol station in Paraparaumu Beach as "a place of interest" in the case and removed items as part of the investigation. The service station manager confirmed police seized car cleaning products and a computer.
Mr Oxnam said today the 4WD was being examined, and the case had been upgraded to a homicide investigation.
"I can now say that we have evidence that Son's death was violent," he said.
Mr Piper's family tonight issued a statement through police saying they were devastated their beloved family member had been killed.
"Son's death leaves a huge hole in our lives. He loves us and we love him. He dotes on his three year old son Cayden who misses him dreadfully."
They pleaded for the return of Mr Piper's body.
"It's very hard for us not knowing where Son is. All we ask is that people who have information about Son's death contact the police. We want Son's body so that we can grieve for him, respect him and lay him to rest."
Mr Oxnam said police wanted witnesses who saw the 4WD after May 15 to come forward.
The vehicle was last seen in the Paraparaumu area that morning, and police know it arrived at Turakina between May 24 and 27.
Mr Oxnam also appealed for the return of Mr Piper's body so his whanau and friends could "have closure and give Son a final resting place".
