Police Scotland has confirmed that the investigation into the deaths of 12 red kites and four buzzards in the Highlands has concluded without anyone being charged.
The incidents occurred in the Conon Brigde area of Ross-shire during March and April 2014.
Police said the case is no longer active due to the three-year time bar for prosecutions on the offences.
The birds were poisoned by pesticides Carbofouran, Aldicarb and Carbosulfan, which are all banned under UK wide legislation.
Police Scotland said they had worked with multiple partners, conducted significant enquiries led by CID officers, undertaken land searches led by Police Search Advisors and carried out by specialist officers from the Operational Support Unit.
Additionally a police dog trained in detecting pesticides was also used during the searches.
A force spokesperson said: "We have also appealed to the community for information to assist with the enquiry and a £27,000 reward for information was offered by partners, however the case remains unsolved."
Divisional Commander Chief Superintendent Philip Macrae said: "Every line of enquiry has been explored as part of our investigation into the deaths of these birds of prey, including large scale searches and a detailed investigation by CID and Wildlife officers.
"We liaised closely with a number of partners including the National Farmers Union, the RSPB and the SSPCA as well as the Crown Office and Procurator Fiscal Service.
"The enquiry has been regularly reviewed for any new lines of enquiry and it is therefore very disappointing that there has been insufficient evidence to progress this case any further.
Detective Inspector Scott Macdonald who led the enquiry said: "I remain convinced, based on the advice of partners involved in the investigation, that the bait was laid for illegal pest control and not specifically to target the birds of prey, however, they became the unfortunate victims of this illegal act.
"Using illegal pesticides is wholly unacceptable; it poses an indiscriminate danger to humans and wildlife alike. Although this case is no longer active, we continue to take the use of illegal pesticides seriously and I would encourage anyone with information to come forward to Police Scotland or anonymously through Crimestoppers on 0800 555 111."
Duncan Orr-Ewing, Head of Species and Land Management for RSPB Scotland said: "This appalling incident should act as a warning to anybody contemplating using illegal poisons in the countryside as to the possible risks to red kites and other vulnerable wildlife.
"We look to individuals and businesses who may still hold stockpiles of banned pesticides to dispose of these chemicals safely following the advice of the public authorities.
"We thank the police for their thorough efforts in investigating this case, and also to all parties who offered up a reward for information leading to a conviction.
"We call on members of the public to remain vigilant and provide any information on the illegal killing of red kites and other raptors to the police."