The theft of more than 140 sheep, including some from a 15-year-old boy, is said to have been an 'organised' theft, according to the police.
The incident happened Rossett, near Wrexham in North Wales on 28 January, with the police calling it the single biggest theft in its five years of operating.
Farmer John Lightfoot had 120 sheep stolen from rustlers, and teenager Jack Sinott had 22 taken.
The sheep are a mix of fat hoggets and in-lamb ewes.
PC Dave Allen, from North Wales rural crime team, said: “People have to have the skill to gather livestock in, and also, where are the livestock going afterwards?
“They must be going somewhere, so it points to organisation.”
15-year-old Jack bought his flock with pocket money saved up after he worked on his grandfather's farm.
“I worked hard for my lambs and I don't know how I could replace them because I'd spent all my money on them this year, trying to look after them so I had the best lambs I could have,” he told the BBC.
“I feel upset and just wish I could have them back.”