Four lambs have been found decapitated in Newent, Forest of Dean over the weekend.
The carcasses had been removed and the heads were left in a field in Malswick.
It is believed that the incident happened overnight between Friday 19 May and 5pm on Saturday 20 May.
There had been 16 sheep in a paddock on a farm, and the others have since been moved to a different location.
Gloucestershire Police are investigating and asking anyone who has information on what happened to come forward.
The public is being asked to call police immediately on 999 if they see suspicious activity and offenders are present or nearby.
Extra police patrols are now being put on and officers from the neighbourhood policing and rural crime teams are liaising with local farmers to raise awareness of the issue.
Information can be provided to police online.