Police have raided the Moy Estate, near Inverness, after birds of prey were found poisoned in the area.
A total of 45 investigators, including wildlife crime officers and staff from the RSPB and Scottish Natural Heritage, have been involved in the operation.
Police have been searching outbuildings and the wider area.
Northern Constabulary said 25 police officers took part in the raids. A number of rare red kites had been killed.
A police spokesman also confirmed that poisoned bait had previously been found on the estate, and that their investigation had been ongoing for a number of weeks.
The carcass of a grouse has tested positive for an illegal poison, the force said.
RSPB Scotland said one of the dead kites had been among birds of prey which were satellite tagged for its Eyes to the Skies schools project.
The conservation charity said suspicions were raised when its signal, which was being tracked via the internet, stopped moving.
Potential evidence has been removed for forensic examination, but searches for further evidence were continuing.
Area commander for Inverness, Ch Insp Andy MacLean, said wildlife crime was a blight on the environment and a serious concern to the public.
He added: "Northern Constabulary consider such crime a serious risk to the safety of the public and have demonstrated today our intention to rigorously deal with reports of wildlife crime, in particular the poisoning of wildlife on open ground which is accessible to the public."
Separately, the remains of a 20-year-old red kite brought to Scotland from Sweden in the 1980s as part of a re-introduction programme have been found in the Highlands.
RSPB conservation manager Kenna Chisholm found the bird while walking her dog near her home in Avoch, the Black Isle.
A ring on one of the bird's legs revealed it was among the red kites re-introduced to the Highlands.
The female raptor was also one of the oldest in the north of Scotland and was believed to have died of natural causes.