A remarkable photograph hangs on the wall of Laurence Rees's office at the BBC. Taken some time during the 1930s, it shows a convivial outdoor tea party, but it is the figures at the far end of the table who seize the attention. All three are gazing with disconcerting intensity at the person speaking. They are Magda Goebbels, her husband Josef - the Nazis' propaganda chief - and, inconspicuously in a corner, Adolf Hitler himself, taking tea and staring.

"People always say Hitler fixed you with his eye contact, looked at you just a bit longer than anyone else," says Rees. He should know: the producer of two of the most memorable - and award-laden - documentary series of the past 10 years, The Nazis: a Warning from History and, earlier this year, Auschwitz: the Nazis and the Final Solution, has almost certainly interviewed more former Nazis than any other Briton alive. It is almost as if he has become transfixed.

It has been his quest for the past 15 years to track them down and persuade them to talk on camera with extraordinary and sometimes appalling frankness, in an attempt to understand what persuaded them to do what they did, why they allowed their country to be taken over by such a man and his cronies, and what they saw in a figure we have always viewed as slightly absurd as well as frightful.

Rees, who started his BBC career as an assistant producer on That's Life, has become one of the most significant interpreters of the Nazi regime. This week he has stepped in front of the camera for the first time to present a new series of extracts from those interviews for the UKTV History Channel.

The interviews have produced some heart-stopping moments, such as when a grandmotherly old woman, confronted with the evidence that she had shopped a neighbour to the Nazis, looks up to the sky and murmurs defensively: "Oh look, it's started to rain ..."

Or, in the Auschwitz series, an interview with the former guard Oskar Groening who insisted repeatedly and seemingly shamelessly that it hadn't been so bad, he'd enjoyed his work, before finally blurting out his reasons for agreeing to be filmed. He had always refused to talk about Auschwitz even to his own family but now he wanted to give witness against the Holocaust deniers, such as friends at his local philately club, who had insisted the camps were all a lie. That was why he said on film: "I would like you to believe me. I saw the gas chambers. I saw the crematoria. I saw the open fires. I was on the ramp when the selections took place. I would like you to believe that these atrocities happened, because I was there."

Many of the interviews took months, even years, to set up. Tracing interviewees sometimes involved trawling court records and military archives; other meetings evolved from personal contacts, referrals and recommendations.

There were many rebuffs along the way, as well as coups. For the Auschwitz series the producers traced a Slovakian woman inmate, Helena Citronova, who owed her survival to a love affair with an SS guard, who found a job for her sorting the clothes of gas chamber victims and even saved the life of her sister, though not her sister's children, as she was about to be shuffled into the showers.

They even tracked down the guard himself, Franz Wunsch, and almost persuaded him to talk too, until his wife refused to let him - not because of the shame of his past, but because of the humiliation of his having loved another woman before her.

Among his interviewees Rees came across Christina Soderbahn, a German film actress, who told him about the "wonderful" eyes of her lover Goebbels and the propaganda chief's former film editor who described appreciatively how brilliant his boss had been in the cutting room.

He interviewed Reinhard Spitzy, adjutant to foreign minister Ribbentrop, who told him: "Hitler was very nice to me, so I can't complain." And Karl Boehm-Tettelbach, who served on Hitler's headquarters staff (before going on to a successful management career with the American airline Pan Am), who unhesitatingly said he still did not approve of the 1944 plot to assassinate the Führer.

Why did the interviewees decide to talk? "I discovered a weird thing," says Rees. "A lot of these Nazis liked the British and could not understand why we were not allies. They even trusted the BBC more than German television.

"My search coincided with the fall of the Berlin Wall and the opening up of the East, freeing up more witnesses. And I was helped by a window in these people's lives. A lot of them had had very good careers - I never met a poor former Nazi - and would not have talked earlier, but I got them at the point when they had retired and before they died.

"I think it must be something to do with your life history having a pattern and at the end of it you look back and remember the things that had a real impact on you. You become reflective about what was really important. And, of course, they wanted to explain themselves to their grandchildren, who were looking at them and saying: 'I love you but I can't understand why you became a Nazi'."

Rees has interviewed Russian veterans and Japanese too, but says the Nazis were different: they were not bullied or threatened with execution if they did not comply. "They tell you it seemed like the right thing at the time. You get into all this stuff about the Allied bombing as if it was a moral equivalent. There is a weird parallel morality. You are sitting interviewing them in their nice house, or a hotel in Munich, and it is like talking to a tribe which is lost. With the Nazis it is not true at all that they were only doing it because they were under threat of being shot. They were doing it because they believed in it.

"I think there's something in people that wants to have faith in a great leader. Hitler wasn't a carpet-chewing madman, at least not until the end. In the 30s they were looking for someone who appeared strong, who did not display uncertainty. The Germans felt they had tried democracy and they voted voluntarily for people - communists and Nazis - committed to removing democratic government. They were very keen to tell me there was a distinction between the 'good' Hitler of the 30s and the 'bad' Hitler of the war."

These were not likeable people - "I can't say I made any friends. I suppose if I had met someone who said, 'I am so sorry', you could have had a relationship. I don't like their equivocation."

Perhaps the worst, he says, was Petrus Zelionka, a Lithuanian who joined the Nazis and admitted to shooting Jewish children and their parents into pits. Asked why he had killed innocent people, he replied that it was because of "a certain curiosity".

Rees asked Groening why children had to be killed at Auschwitz; the former glass factory personnel manager replied: "The enemy is the blood in them. The enemy is their growing up to become a Jew who could be dangerous."

Rees's second world war series have been shown to acclaim around the world, not least in Germany. He says there has never been much feedback from the interviewees - most of whom are now dead. No complaints, and at least the programmes are not being shown at veterans' reunions. Before the Auschwitz series was shown in January a BBC poll indicated that 60% of British women under 35 had never heard of the place. They know now. One episode attracted 4 million viewers, more than Celebrity Big Brother on another channel.

But on the wall outside Rees's office hang a couple of messages to him: an article written by him, torn from the Mail on Sunday by someone who had ranted in spidery capitals all over it about how the Nazis had the right idea about Jews, Gypsies and homosexuals. Next to it an anonymous letter calls him Satan's scum and asks why he didn't notice that the Jews, not the Germans, were the perpetrators of the concentration camps.

· Laurence Rees presents Inside the Nazi State this week nightly on UKTV History at 9pm.