A golden eagle that was found dead in Angus earlier this month was poisoned, tests have revealed.
Police said the bird of prey was discovered in the Glen Lethnot area after satellite tracking equipment indicated it had stopped moving.
Officers have appealed for anyone who was in the area between 10-25 November to contact them.
The force said it was also conducting searches in an attempt to uncover any poisonous bait.
The eagle, named Fearnan, was ringed as a chick in a nest near Loch Tay in Perthshire in June 2011 and had spent much of its life in Badenoch, before moving to the Angus glens in early November.
Just three weeks later, it had been poisoned.
Stuart Housden, director of RSPB Scotland, said: "Incidents such as this show very clearly why this iconic bird needs not just our recognition, but also greater protection. We sincerely hope that those responsible are swiftly brought to justice and would encourage those with information to come forward."
In the past five and a half years, another four eagles, a red kite and seven buzzards have been shot, poisoned or trapped on sporting estates situated in the Angus Glens, RSPB Scotland said.
In January 2013, the nest tree of a pair of white-tailed eagles was felled. No-one has been prosecuted for any of these offences, according to the charity.
Mr Housden added: "I will be asking the environment spokesperson of all the parties in the Scottish Parliament to take cross-party action to stiffen the penalties for those convicted of such offences and to look again at the regulation of sport shooting. The current state of affairs is simply unacceptable."
A recent report by RSPB Scotland revealed that a significant number of incidents of illegal killing of birds of prey took place in areas managed for driven grouse shooting.
PC Blair Wilkie, Wildlife Liaison Officer for the Angus area, said: ''Given the rural location of this crime, we appeal to anyone who was out walking, working, or indeed out on the hills for whatever purpose between those dates, to get in touch.
"You may not think that any information you have is of value to us, but please let us be the judges of that. It's also important to stress to the public that in cases where poisoned baits are used to target birds of prey, the poisons present a wider threat.
"They are, without question, a significant health risk to both humans and animals. If you find what you suspect to be poisoned bait - do not touch it."
Scottish Environment Minister Paul Wheelhouse said he was "disgusted and angry" by what appeared to be a "deliberate, illegal and cruel action against a protected species."
He added: "It is a great regret that such actions undermine the reputation of sporting estates at a time when there has been genuine progress on the part of the majority of estates and gamekeepers.
"The actions of a small and ignorant minority are extremely damaging and the Scottish public will rightly be outraged by this news and I share that sense of outrage.
"Crimes like this, and the buzzard found recently near Stirling, strongly reinforce the need, and give a very clear justification, for the measures I announced in July this year.
"I have said before that whilst there has been progress we will continue working to put those measures into practice, but there is more work to be done to tackle wildlife crime in Scotland. Police Scotland are carrying out enquiries and I will await any developments in respect of this case with great interest."
Douglas McAdam, chief executive of Scottish Land and Estates, said the organisation was "appalled" by the eagle's death.
He added: "Scottish Land and Estates is an enthusiastic member of the Partnership for Action Against Wildlife Crime Scotland and will continue to do whatever it can to help the police and the Scottish Government, working in partnership with relevant agencies and organisations, to eradicate wildlife crime in all its forms and we wholeheartedly condemn the mindless minority who commit such acts."
A spokesman for the Scottish Gamekeepers Association said news of the poisoning was "terrible", particularly given the "significant progress made in recent times".
He said: "We would like to reiterate that the SGA will not condone the poisoning of birds of prey. If any SGA member is convicted of such a crime, they are removed from our organisation.
"We will not allow the actions of a minority to tarnish the reputations of the vast majority of sporting estates who manage responsibly."