Officers investigating the death of two horses say the animals showed no sign of external trauma and they may have died of natural causes.
Post mortems have now been conducted and there was no evidence that the horses had suffered any sort of abuse.
Police are now awaiting a toxicology report and say they are keeping an open mind as to how the horses may have died.
A member of the public called officers to a field off Quarry Hill Road, Ilkeston, on Sunday afternoon, after discovering the animals.
Anyone with information about these deaths should call Derbyshire Constabulary on 101 or Crimestoppers on 0800 555 111.