“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,” said Detective Chief Superintendant Julian Gregory in a statement.

The probe, which involved multiple law enforcement branches, was conducted under the Computer Misuse Act 1990, which sets a three-year limit on criminal proceedings from the date of the initial offense, police said.

Gregory said the cops’ probe shows that the breach stemmed from a “sophisticated and carefully orchestrated attack on the CRU’s data files, carried out remotely via the internet.”

It wasn’t an inside job, the Norfolk Constabulary said.

“There is no evidence to suggest that anyone working at or associated with the University of East Anglia was involved in the crime,” Gregory said.

The emails were unveiled in mid-November of 2009.

That was just weeks ahead of the big United Nations’s climate change conference in Copenhagen, a fractious affair that disappointed climate advocates by ending with a weak agreement that was not even formally approved.