The number of birds of prey found dead within a two-mile area has risen to 13, with poisoning confirmed in some of the cases.
Nine red kites and four buzzards have been found in the last week near Conon Bridge in the Highlands.
Police, the RSPB, the Scottish SPCA and experts from the Scottish Agricultural College are working to investigate the wildlife crime.
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A police spokesman said post-mortem examinations are ongoing but poison has been detected in "several of the birds". Further tests are to be carried out on the rest of the raptors.
All of the dead birds have been discovered in a two square mile area to the south east of Conon Bridge around Conon Brae, Balvail, Leanaig and Alcaig.
The spokesman said: "Following multi-agency searches, which initially began on Monday March 24, further dead birds have been recovered increasing the total to that of 13 birds of prey, consisting of nine red kites and four buzzards.
"Post-mortem examinations of the recovered birds is ongoing but the presence of poison in several of the birds has already been established. The presence of poison confirms our early assessment of criminal involvement in the deaths."
Police are appealing for any witnesses who have seen anything suspicious in the area.
Officers are also advising anyone who finds any further dead bird of prey to note its location and inform them, without attempting to recover it.
The RSPB described the deaths as "shocking" and is considering offering a reward for information.
Duncan Orr-Ewing, head of species and land management at the conservation charity, said: "This is devastating news and confirms our worst fears. This appalling incident highlights the very real threat illegal poisoning poses to fantastic species like red kites.
"The vulnerable Black Isle population in particular has been repeatedly hit by deaths due to illegal poison use. The Chilterns population in southern England is nearly 10 times bigger, yet both projects started at the same time in 1989 with the same number of birds released into the wild.
"That is a shocking indictment on behaviour of some in this part of rural Scotland.
"We urge anyone with information relating to this incident to contact Police Scotland so the perpetrators can be identified and brought to justice. We are considering offering a substantial reward for information that leads to a successful conviction and will be discussing this with the police."
Scottish Gamekeepers Association chairman Alex Hogg said: "The discovery of so many birds in one area is unprecedented and alarming.
"Police Scotland deserve as much help as possible as they try to deal with the situation so we continue to encourage people to help them if they know anything.
"The indiscriminate use of poison is unacceptable and condemned by the Scottish Gamekeepers Association."
The deaths come as figures for 2013 showed that six birds of prey were illegally poisoned last year.
A red kite, a golden eagle and four buzzards were poisoned in 2013, double the number for the previous year but well below a peak of 30 in 2009.
Figures from the Partnership for Action Against Wildlife Crime (PAW) Scotland reveal there were a total of 15 recorded crimes against birds of prey including shootings, trappings and nest destruction last year.
The RSPB said the figures are ''very worrying'' and show that birds continue to be persecuted in the Scottish countryside, whether by deliberate or accidental means.
A map published by PAW Scotland shows the location of all recorded crimes against birds in 2013. Two of the poisoning cases were discovered in Perthshire, two near Stirling, with one in Angus and the other south of Edinburgh.