No criminal charges will be brought despite evidence of a "sophisticated and orchestrated" hacking attack on a university's computers, police said today.
Norfolk Police launched an inquiry after hacked emails from the University of East Anglia's Climatic Research Unit (CRU) were leaked online in 2009 before crunch UN climate talks.
Climate change sceptics claimed the emails showed scientists manipulating data to support a theory of man-made global warming, but a series of reviews later cleared researchers at the unit of any scientific impropriety.
Today the force said that the investigation had concluded the breach was "the result of a sophisticated and carefully orchestrated attack on the CRU's data files, carried out remotely via the internet".
But a statement added that there would be no criminal proceedings.
Detective Chief Superintendent Julian Gregory said: "Despite detailed and comprehensive enquiries, supported by experts in this field, the complex nature of this investigation means that we do not have a realistic prospect of identifying the offender or offenders and launching criminal proceedings within the time constraints imposed by law.
"The international dimension of investigating the world wide web especially has proved extremely challenging.
"However, as a result of our enquiries, we can say that the data breach was the result of a sophisticated and carefully orchestrated attack on the CRU's data files, carried out remotely via the internet.
"The offenders used methods common in unlawful internet activity to obstruct enquiries.
"There is no evidence to suggest that anyone working at or associated with the University of East Anglia was involved in the crime."
The investigation, code-named Operation Cabin, focused on unauthorised access to computer material, an offence under the Computer Misuse Act 1990, which has a three year limit on proceedings from the commission of the original offence.
A statement said: "It has been concluded by Norfolk Constabulary, in consultation with The Met, that due to outstanding enquiries this is now an unrealistic prospect."
Assistant Chief Constable Charlie Hall said: "Online crime is a global issue.
"While law enforcement agencies continue to develop our response to emerging threats, it falls upon individuals and organisations to be alert to this and take steps to mitigate risk as far as is practicable."
Professor Edward Acton, vice-chancellor of the University of East Anglia, said he was disappointed that those responsible for the crime would not be brought to justice.
He added: "We are very grateful to Norfolk Constabulary for their sustained effort over the last two and a half years, and appreciate the difficulty of devoting continued resources to such a complex international investigation. Clearly the perpetrators were highly sophisticated and covered their tracks extremely carefully.
"The misinformation and conspiracy theories circulating following the publication of the stolen emails - including the theory that the hacker was a disgruntled UEA employee - did real harm to public perceptions about the dangers of climate change.
"The results of the independent inquiries and recent scientific studies have vindicated our scientists, who have returned to their important task of providing the best possible scientific information on this globally critical issue."
Prof Phil Jones, research director of the CRU, also thanked the police for their work on a "difficult investigation".
He added: "I am obviously disappointed that no-one has been prosecuted for this crime but hope today's announcement will draw a line under the stressful events of the last two and half years. "My colleagues and I remain committed to the research CRU undertakes to illuminate the globally important issue of climate change."