A police chief claims that farmers are "probably" to blame for the thefts of thousands of sheep in Powys.
Sheep worth about £400,000 have been stolen since 2004
Chief Inspector Steve Hughson, of the Dyfed-Powys force, said evidence pointed to those who knew how to work dogs and had local knowledge.
He urged law-abiding farmers to come forward, adding that they had to know who was stealing their animals.
Nearly 8,000 sheep, worth about £400,000, have been reported stolen in Powys since 2004, according to police.
Meanwhile, one farmer has offered a £5,000 reward after losing 1,000 sheep to thieves.
Officers suspect his animals, and the thousands of others stolen during the last three years, have supplied the illegal meat trade.
However, they said reported thefts had fallen thanks to Operation Jasper, a joint police and trading standards campaign which has been running for three years next month.
A dedicated officer and a council official have joined forces with other agencies to clampdown on the rustling problem.
However, police admit it is still at a worrying level, and that their task is huge.
Powys is one of the largest counties in the UK and has 5,400 farms, and the majority have sheep.
Chief Insp Hughson said: "The sort of person who steals livestock has got to know how to do it. The average person wouldn't have a clue.
"It's probably fellow farmers with dogs who are responsible. Farmers must have an idea who is stealing their sheep and they need to have trust in the police and the confidence to come forward."
One farmer, who did not want to be named, said he had lost more than 1,000 sheep to thieves since 1989.
He has put up a reward of £5,000 for the successful conviction of those responsible, and he is in no doubt that rogue farmers are to blame.
Nearly 8,000 sheep have been stolen since 2004
"Since 1989 I have lost more than 1,000, worth about £20,000," he said.
"I claimed on the insurance for the first couple of years, but then my insurance company said it wouldn't keep on paying out so I've taken the thefts on the chin.
"Some might question why I keep sending my sheep up onto the hill in Powys (in the Cambrian Mountains), but I rent 1,000 acres and must use the land.
"I suspect farmers, or people with the knowledge of working dogs, are to blame. They must have local knowledge too."
People with information can contact Pc Charlie Jones at Rhayader police on 0845 330 2000 or Michael Davies at Powys trading standards on 01597 826033.