By Dominic Casciani
BBC News at the National Archives
Faced with multiple threats in the middle of wartime, what exactly should a nation's security services do if a small boat arrives out of the blue at a remote port, carrying a man claiming he has come to spy on the enemy for Britain?
Arnold Evensen: Innocent or a spy?
While the public image of spies and intelligence agencies is that of glamorous locations and James Bond gadgets, the reality is often painstaking back-room analysis and, very often, judgement calls requiring the steady nerve of a poker player.
MI5 files relating to a curious incident at the height of World War II reveal how difficult this job can be - and the lengths that the organisation went to to work out whether or not a certain Arnold Evensen, former Norwegian baker and army cook, was a friend or foe.
It was on 8 January 1943 that Evensen and two other men, Louis Westrum and Gunnar Bjorn Pedersen, brought the MV Reidar into dock in Lerwick on the Shetland Isles.
Evensen declared that he wanted to aid the war effort, that he was someone who could be used to gather intelligence on German activities in Scandinavia.
By way of an opening offering, the 28-year-old handed over log books and what he said were covertly-taken photographs of German armaments in Trondheim, and warships at anchor in the Fjords.
However, the British had been expecting the arrival having received advanced warning the Germans were considering a double-cross plot to get agents into the UK via boats from Norway, posing as refugees fleeing the Nazis.
The three were taken to Camp 020, MI5's secret centre in south-west London, where the British were perfecting interrogation techniques still used around the world.
So what was the truth?
According to the MI5 papers released to the National Archives, Evensen said he had been recruited by an old friend, Arfinn Hegdahl, who had become a confidante of the German intelligence services.
Hegdahl, claimed Evensen, was no traitor and wanted to find ways of undermining the occupation.
When his German handler suggested running boats to the Shetlands, Hegdahl saw it as a means to achieve his aims, claimed Evensen.
He then recruited another old friend, Ole Svensen, and the men, working in various ways, set about constructing an elaborate double-cross to fox the Germans and aid the British, he claimed.
Part of the plan appeared to revolve around a bluff centred on two radio sets that could be taken into Norway. The first would be known to the Germans and used to send fake intelligence reports back across the North Sea. The second secret set would be used to send real information back to London, proposed Evensen.
But the story, and in particular the pictures, raised eyebrows.
A month into the interrogation, MI5 had virtually concluded that the other two men were entirely innocent (they were later released). But in Evensen's case, the only thing they were sure of was that he had worked for the Germans.
While interrogators continued to question Evensen, analysts pored over the images of warships and guns with equal effort.
And they concluded they were not to be trusted. The Germans would simply not allow Norwegians to come close enough to the heavy artillery guns to take pictures of this calibre.
"To my mind, this leaves one with a very strong impression that these photographs were a deliberate plant," wrote one official.
"The position of Ole Svensen, from whom the photographs are alleged to have been obtained, requires close investigation."
So was Evensen a German spy attempting to pull off a dramatic triple-cross?
Fog of suspicion
After three months of questioning, MI5's enormous file included interviews and rambling statements by Evensen which read like a who's who of Trondheim life.
"The Camp 020 interrogators maintain that Evensen ought not to be judged by any ordinary standards as it is obvious that his mental powers are sub-normal, his memory hopeless and his mind an inchoate jumble," said the final report.
"His story is in parts quite incredible and prima facie there appears to be a strong case against him. They however maintain that after close interrogation and observation of the man for three months, the fog of suspicion almost entirely disperses.
"Evensen's brain is so confused and his memory so bad that he could never lie successfully, and the fact that no important discrepancies occur in his various statements must be regarded in his favour.
"The real problem is to know what to do with him. He himself still seems to be under the impression that he could be utilised in some unspecified form of espionage in Norway.
"This would of course be quite out of the question. Instead I suggest that he be employed somewhere in this country where he may be under observation and that desire for excitement of his, which has caused so much trouble, may be curbed. "
Evensen was freed - but denied the chance to be a spy. He became a baker in west London.