An investigation was launched after a family dog was found dead on a footpath with heroin in his bloodstream.

Animal welfare charity, the RSPCA were pulled in to investigate the shocking cruelty after Benson was found dumped near a children’s park in southeast England.

A postmortem confirmed the Bull Mastiff’s body contained the class-A drug but the cause of death remains a mystery.

The seven-year-old Bull Mastiff cross “wouldn’t hurt a fly” and was “scared of other dogs” according to his owners who were traced by his microchip.

Benson had a bloodied face and his body was already starting to decompose when he was discovered near a footpath still wearing his lead and a harness.

He had been dead for several days when he was found in Lewes, East Sussex, on November 21 and his body was taken to a local vet for storage — but his owners refuse to say how long he was missing for.

Determined to get to the bottom of his death, residents launched the “Justice for Benson” campaign and lobbied the RSPCA to investigate.

“We would like to thank the people who have supported the investigation into the death of poor Benson and especially those who contributed to the cost of the postmortem,” the RSPCA said in a statement.

“The pathology report confirmed Benson’s body contained a class-A drug but the actual cause of death could not be determined due to the condition of his body.

“Our inspector has been investigating Benson’s death for a number of weeks, but sadly due to lack of evidence to explain the precise circumstances of this matter, we have been unable to proceed further at this stage.”

Dog charity trustee Melanie Beck was among campaigners fighting to give “Benson a voice.”

“We are incredibly grateful to the RSPCA for all their hard work with this — I don’t believe we have failed either though,” she said.

“We and the 1200 people following his story gave Benson a voice and that has to count for something.”

Beck said it was “incredibly disappointing” that they hadn’t been able to get “Justice for Benson” at this time, but that the RSPCA has vowed to “reopen the case if more evidence comes to light.”

A GoFundMe page was set up in honor of the dog, which has raised more than £1,200 ($1,590) from almost 100 generous donors.