Two white-tailed sea eagles fitted with satellite tags have disappeared, according to RSPB Scotland.
The charity said one of the young birds was last recorded near Inverness and the other in Aberdeenshire.
The RSPB Scotland said transmissions from both tags ended on 22 July in areas managed as grouse shooting moors.
The Scottish Gamekeepers Association said tags could fail, but urged anyone with information about suspicious bird deaths to contact police.
Landowners organisation Scottish Land and Estates said owners of land in both locations were willing to help in searches for the eagles.
Both missing birds were from a small white-tailed sea eagle population in east Scotland.
RSPB Scotland fears the eagles have been illegally killed.
Head of investigations Ian Thomson said: "The disappearance of these two eagles is more than a loss of two birds.
"It means any future breeding success they might have had, helping to boost the numbers of these rare birds, has also been destroyed.
"Illegal persecution is seriously undermining the re-establishment of a white-tailed eagle population in this part of Scotland."
The Scottish Gamekeepers Association (SGA) said it had concerns about the reliability of satellite tags and also that grouse management were being blamed for the loss of birds of prey.
A spokesman said: "The SGA has petitioned the Scottish Parliament for independent monitoring of these tags so all of these possibilities can be properly explored by neutral tag experts, appointed by Scottish government.
"We would reiterate that if anyone sees these birds that they contact Police Scotland immediately."
Sarah-Jane Laing, executive director at Scottish Land and Estates, said it was disappointed the "finger of blame" had been pointed at grouse moors.
She said: "We wholeheartedly condemn any form of raptor persecution and firmly support tougher penalties for those found guilty of such crimes.
"It is frustrating, however, that more than three months has passed before the appeal for information has been issued publicly by RSPB and we await further information from Police Scotland."