The racing industry has been thrown into turmoil after David Smith was accused of killing up to 10,000 dogs and burying them near his £220,000 detached house in Northdene Terrace.
The dad-of-three, who runs a builder's merchant and newsagent's in Lord Street, charged £10 a time to shoot dogs, allegedly for 40 trainers, because they were too old to race and too expensive to home.
Animal rights campaigners said they believe most of the slaughtered dogs would have been raced at Sunderland and Brough Park stadiums – both owned by bookmaking giant William Hill.
"William Hill knew about this," said Tony Peters, of Greyhound Action. "They've know for years this guy Smith has been killing dogs that come from their tracks."
The firm denied the allegations.
A spokesman said: "William Hill stadia hosts a responsible and regulated sport and has no reason to believe that any owners or trainers from our tracks have been involved with this activity.
"We find the revelations abhorrent and will welcome and support any investigation into the activity to identify any miscreants who may have chosen to euthanase their greyhounds by these means.
"Should an investigation identify any individuals have raced greyhounds at our tracks and who have then engaged in this activity, these individuals will be banned from having any further association with our tracks immediately and will be reported to the NGRC (National Greyhound Racing Club, which regulates 31 licensed tracks)."
Durham police have twice spoken to Mr Smith since revelations of his activities came to light over the weekend
A spokesman said: "We have received no specific complaints about Mr Smith's activities and we have no concerns about the bolt gun and we have now established it is quite legitimately held."
On whether the alleged mass dog grave posed a public health hazard, District of Easington Council said: "We will be working with the Environment Agency to ensure there will be no long-term contamination of the land at Mr Smith's home.
"There are potential health issues surrounding the disposal of animal carcasses and we would therefore strongly advise people to use pet crematoriums or contact the council for further advice."
The NGRC said its North East stipendiary steward Eric Vose would investigate.
But Greyhound Action fears any investigation may be worthless unless the one-acre plot where Mr Smith allegedly buried 10,000 dogs is dug up.


Death threats to dog killer

DOG killer David Smith has been forced to boost security at his Seaham home after receiving death threats.
Since the allegations were made against the businessman, malicious phone calls have been made to his businesses and Northdene Terrace home.
Some of the sinister calls were answered by Mr Smith's wife Maureen and his daughter.
To protect his property and family, Mr Smith hurriedly installed CCTV, floodlights and other security measures after the allegations received national attention.
"I am devastated by all of this," said the East Durham builder's merchant.
Durham police said they were not aware of any death threats, but confirmed the Smiths had received a number of malicious phone calls.


Killings 'common knowledge'

PEOPLE in Seaham have backed David Smith, who has received death threats for allegedly killing thousands of greyhounds.
Many people said it was common knowledge dogs were being put down.
"Everybody in Seaham knows what he does" said Frank Prest, of Seaham Pet and Garden Centre. "And have done for a long time.
"As long as he's doing it humanely, he's providing a service. If he didn't do that, they would just smack them over the head with a brick."
Deborah Rochester, 29, from Ropery Walk, said: "I don't agree with the methods he's using but I can't see why everyone is putting up a fuss about it now because he's been doing it for years."
Her friend Angela Peel, 30, from Dawdon, said: "He's not doing anything illegal and people have been using him for years."
They both said greyhound owners should take more responsibility for their dogs.
Pauline Yates, 41, from Seaham, agreed. "It's up to the owners to look after them until they die," she said.
Fifty-year-old Alan Savage, from Parkside, also said blame for the situation should lie with the owners.
"It should be the owners held up as well, not just Dave Smith. They're the ones making money off the dogs then just getting rid of them."


Racing told to clean up its act

THE greyhound industry has been told to "clean its act up" by the Government.
Animal Welfare Minister Ben Bradshaw said killing a dog with a bolt gun breached the rules of the National Greyhound Racing Club.
"The National Greyhound Racing Club must launch an immediate investigation," he said.
"It must discipline or expel any members who have broken its rules by disposing of their dogs in this way."
"The Government believes racing greyhounds should only be put down by a vet.
"We have long felt self-regulation within the greyhound industry is the most effective way of policing animal welfare.
"If they can't clean up their act, Government will intervene."
Meanwhile, campaigners have called for commercial greyhound racing at stadiums such as Sunderland and Easington to be scrapped.
Greyhound Action says replacing real dogs with computer-generated virtual racing is the only way to ensure thousands of greyhounds a year do not end up prematurely killed.