In its first year of operation alone, the Historical Enquiries Team (HET) had a turnover of 40 per cent, with close to a third packing in the job the following year.

Politicians last night questioned why so many people were leaving within a year of joining the team.

HET has been appointed to investigate around 3,300 deaths, of which around 2,000 were republican murders and 1,000 committed by loyalists.

The security forces were involved in incidents which resulted in the killing of under 400 people.

HET director Dave Cox told the Northern Ireland Affairs Committee: "With people living away from home in the week, coming to Northern Ireland, living in lodgings, going home on a Friday, we tend to find that after about a year or so many of them leave."

DUP victims' spokesman Jeffrey Donaldson called for a closer look at the high turnover.

"Whilst I believe the leadership structure in HET is very sound the detectives who took on these jobs would have known they would be spending time away from home when they signed up," he said.

"I think we should be examining their reasons for leaving closely to find what the underlying factors in their decisions may be."

Party colleague Sammy Wilson, a member of the NI Affairs Committee, said he found the turnover rates "quite alarming".

He said: "I suppose some of the detectives concerned thought they were going to be reinvestigating crimes and bringing people to book, but then found out that is not quite what they were doing."

He was concerned about the "immense" amount of resources involved and the fact that the HET has only questioned one terrorist under caution to date, despite having completed reviews of over 400 murders and having more than 1,000 under active investigation.

In contrast, it was revealed last week that hundreds of police officers are likely to be quizzed by the Police Ombudsman about their conduct in relation to some 300 historical deaths currently on HET's books.

Victims' campaigner Willie Frazer said there was "more to the high staff turnover (of HET detectives] than meets the eye".

He added: "I believe these people have come in and not been able to do their jobs properly and then just got up and left. People here aren't stupid."

An HET spokeswoman said many employees, often retired detectives from Great Britain, only signed up to the job for a limited period because of the travel.

"However, the nature of the work is also very challenging and some people find that after an extended period, they do not want to experience any more of the trauma involved in these cases," she said.