Ornithologists fear "something catastrophic" has happened to an "extremely rare" bird of prey after its satellite tag stopped transmitting.
The Montagu's harrier is one of only seven breeding pairs in England and is legally protected.
The device stopped transmitting on 8 August. Its last known location was near Great Bircham in north Norfolk.
The RSPB said tag failure had been ruled out. Norfolk Police said they were investigating the disappearance.
The BBC's One Show's Mike Dilger filmed the bird being tagged by the Dutch Montagu's Harrier Foundation, which is based in eastern Groningen in The Netherlands.
Information from the tracking device was being downloaded daily and researchers were hoping to find out more about the bird's movements in northern Europe and during its migration to Africa.
The Dutch team is liaising with the RSPB and Mr Madge said: "We are maintaining an open mind and not ruling anything in or out, except the team have confirmed tag failure isn't an issue.
"But we suspect something catastrophic has happened to this bird and that could be illegal persecution or predation."
- It migrates between northern Europe and Africa
- The furthest north it travels is East Anglia and southern England
- It eats small birds, voles, shrews, rabbits, lizards and insects
- It is an extremely rare breeding bird in England
- Its numbers have reduced from 30 pairs in the 1950s
- It is 43cm to 47 cm (17in to 18.5in) long
Mr Madge added: "We have no evidence the bird was harmed, no body has been recovered, no feathers found.
"The other possibility is predation by a fox and and its body was taken underground, which would account for the abrupt end of transmission."
The three-year-old bird had produced a brood and Mr Madge said he did not yet know the fate of its chicks.
Norfolk Police are appealing for witnesses.