It is an unassuming bungalow in a nondescript village on the edge of a small town in southern France. An unmade driveway runs from the road between similarly recent modest villas to the bars of a large electric gate.

The house beyond is spare but comfortable; three plump cats roam the terraced gardens, by some margin the house's most attractive feature. The floors are tiled, the sofas shiny, the television new but not flashy. That's where he saw the news.

"We had a call from Norway," he recalls. "They said, something's happening in Oslo, in the government district. We turned on the TV; we have BBC and Sky here, no Norwegian channels. We sat and watched. Glued, of course."

All that afternoon and evening, 22 July, nobody could say who it was. Eight people had died in the bombing and a far larger number – it would ultimately be 69, mostly teenagers attending a Labour party youth camp – in the subsequent mass shooting on Utøya island.

"They did not know who was responsible," he says. "They were guessing. An Islamist? Then they started to say, a typical Norwegian. Tall, blond. They did not know who. We went to bed. It was late; we were upset. This was our home country."

It wasn't until the next morning that he turned on his computer and saw that the man who had carried out the bloodiest massacre in modern peacetime Europe had been captured, and that his name was Anders Behring Breivik, his son.

He is not a large man, Jens Breivik. Cautious. Precise. White hair, steel-rimmed specs, sober jumper; ask casting for a 76-year-old retired Norwegian diplomat and you surely wouldn't get better. Visibly ill at ease, for at least the first hour.

"I was so … shocked, I did not know what to do," he says. "I couldn't … I was unable to do anything. I sat with my head in my hands. It was a terrible moment. I just could not face it. The media got here that evening and I hid. My wife told them I was in Spain."

Several months later, with his son in the dock – first judged criminally insane, now declared sane enough to face trial and jail – he still feels "terrible. Such pain. Constantly, I am reminded who I am. In the first few weeks, I thought seriously of taking my own life. I've lost the retirement I always imagined; that's gone. I will forever be asking how a man could possibly develop such thoughts. And could I have done something?"

It may not, of course, have made the slightest difference. But could he?

Others, to Breivik's distress, have not been slow to suggest that he could. Assorted commentators have called him "selfish", "narcissistic", "a terrible father". Katharine Birbalsingh, who describes herself as "Britain's most outspoken and controversial teacher", told the Telegraph she was certain his failings had sown the seeds of his son's madness.

It is, to be fair, not a very straightforward story. Nor, maybe, for those who wish to judge such things, a very admirable one. For Jens Breivik, sitting stiffly at his dining-room table confronting roads travelled and turnings taken, it is certainly not easy to tell.

He and Wenche Behring had been together for two years when Anders was born on 13 February 1979. Both had been married before: Jens, then economic affairs counsellor at the Norwegian embassy in Lancaster Gate (his second tour in London), had three children from his first marriage, which had lasted nearly 13 years; Behring, a nurse, had a young daughter, Elisabeth, with her previous husband, who was Swedish.

The couple separated within a year of Anders's birth. "I don't think," says Breivik, cautiously, "she was really interested in marriage. She was an … unusual person. I think what she wanted to be was a single mother. She just left, anyway, went to Oslo with Anders and her daughter. Didn't want me to see my son. You get help in Norway, as a single mother."

(Behring, who later would marry a Norwegian army captain, has never given her side of this story, consistently refusing all media interviews. In a conversation with the psychiatrists who evaluated Anders, leaked to the Norwegian press, she has said only that she first noticed signs of her son's "paranoid delusions" in 2006.)

Breivik senior, meanwhile, stayed in London. Behring reluctantly brought the infant Anders from Oslo to see him, staying at one stage for several months even though the marriage was beyond repair. Then in 1983, Breivik got married, for a third time – to Tove, a colleague – and was posted to the OECD in Paris, subsequently transferring to the Norwegian embassy there.

Anders liked Tove, his new stepmother, and in fact stayed in touch with her until just before the attacks. But soon after the couple arrived in France, Breivik says, it became clear that Anders, now four, was not faring well in Oslo.

"There was a formal report, in 1983, from the Norwegian childcare authorities," he says. "They recommended he should be moved. They said his relationship with his mother, her emotional incapacity to care for him, made it harmful for him to stay. But it was very difficult; Wenche would not admit to any problems. She wouldn't talk to me."

Breivik and his wife applied through the Norwegian courts for custody, hoping the report would work in their favour. It didn't. "This I do not understand, and nor do many people in Norway," Breivik says. "There was an official report saying my son was being harmed by living with his mother. But in Norway, the presumption is always with the mother."

Despite that ruling, father and son appear to have got on fairly well when Anders was still a young child. "In Paris, he visited quite often," Breivik says. "He travelled as an unaccompanied minor; I'd meet him at the airport." Anders stayed at Breivik's embassy apartment, on rue Spontini in the 16th arrondissement; there were summer holidays at a cottage in the Normandy countryside, 10 minutes from the sea at Cabourg.

Anders describes this period in his 1,500-page online "manifesto", remarking that he had "a good relationship with [his father] and his new wife at the time, until I was 15." His upbringing was "privileged", he wrote, in "a typical Norwegian middle class family", with "responsible and intelligent people around me … and no negative experiences" (although he now regretted a "lack of discipline".)

In 1990, Breivik returned to Oslo. "We had what I think anyone would call a normal relationship between a divorced father and his son," he confirms. "He came to my house several times a week, and at weekends. I had a small chalet in southern Norway; he stayed there often, too." There was a trip to the Tivoli amusement park, in Copenhagen, when Anders was 13.

How does he remember his son at that time? Breivik considers. "An ordinary boy. Maybe … not quite ordinary. He was never very communicative; quite withdrawn. He wouldn't talk about his mother, home, school. He came to my place to relax, have a good meal, then – when he was a bit older – to go out afterwards into the city centre to meet his friends."

But by this time Breivik's marriage to Tove was breaking up, too. He is, understandably, reluctant to talk about this; three failed marriages reflect well on no one. This one finally collapsed, he says, when he asked her to contact Alcoholics Anonymous.

Then in 1992 he met Wanda, his fourth and current wife; they married three years later. Wanda "saved my life. Really. I was in a bad way when I met her. Three marriages, three divorces. Wanda's strong. She's helped put me back together. She's helping me through this, too. Though I'm not sure, frankly, that either of us will ever truly get through it."

With his marriage to Wanda, however, the children from Breivik's first marriage decided they wanted nothing further to do with their father. "They're angry with me," he says, flatly. "They think I have made too many … mistakes. Done too many stupid things."

Anders, too, cut loose around the same time, in 1995. Over the previous two or three years, things had become increasingly difficult. In his manifesto, the killer blames his father for the estrangement, saying Breivik "isolated himself when I was 15. He was not happy with my 'graffiti' phase from 13 to 16. He has four children, but has cut off contact with all of them. So I think it is pretty clear who is at fault."

Breivik disputes this. "It's true I was angry," he says. "Several times the police called me to say he had sprayed buildings, trains, buses. He was also shoplifting. But I was always willing to see him, and he knew that. It was Anders who cut it off. His decision, not mine. He was 16, building his own life. He had his hip-hop, too."

Wanda says that the couple saw Anders "regularly" before he finally disappeared. "We invited him to supper, once a fortnight," she says. "I tried with Anders; I really tried. I knew about teenage boys, I knew what interests them. He was always: don't know. Don't care."

Whoever took the initiative, father and son met for the last time in 1995. "He borrowed a jacket from me for his confirmation," says Breivik. "He told me he aimed to study in the States, on an exchange. When I heard no more from him, I thought that was what he had done." Breivik kept sending money, some £200 a month, to Anders's mother.

The two were in contact, briefly, just once more. In 2005, Breivik had a phone call out of the blue. "He told me he was doing well," Breivik says. "He had his own company, data processing, two employees. He didn't want anything; he was just anxious to tell me he was doing well and was happy. I had health problems; I said I was pleased to hear from him, and we should stay in touch. We never did."

In his manifesto, Anders claimed the business was the first step in a nine-year plan leading to the 22 July attacks; a front "for the purpose of financing resistance/liberation-related military operations". Subsequent police inquiries have shown much of this to be delusional – fabulation or wild exaggeration.

This trial will, perhaps, shed some light on what so warped Anders Behring Breivik's perceptions that he was prepared to slaughter 77 of his fellow countrymen in order to "save Norway and western Europe from cultural Marxism and a Muslim takeover".

But in his modest bungalow in France, Jens Breivik lives haunted by the part he may have played in the creation of a monster. Last month Norwegian police, assisted by French officers, spent nearly 13 hours interviewing him in Carcassonne.

The psychiatric report on his son makes clear, he stresses, "that I could have done nothing to prevent what happened." Moreover, he's convinced "I really did all I could when he was small." Maybe, it's true, he could have tried harder to stay in touch later, after 1995.

"But I honestly thought he was okay. Quiet, awkward, but not … abnormal. If he didn't want to see me, there wasn't really much I could do. I had no leverage. And anyway, after that he seemed successful, with his own business, employees. That was good, wasn't it?"

Yet however much he protests, however much he tried or didn't try, Breivik's regrets, one senses, run deeper. He knows his choices have not always been the wisest. Of his relationship with Wenche Behring, he now says: "I was stupid not to see I was being used." His third marriage, to Tove, embarked upon while the wreckage of the second was still smouldering, was also "not perhaps the right step".

In a phrase in his manifesto that, for once, might just come somewhere close to the truth, Anders sums his father up as "just not very good with people". While the photographer is busy with Breivik outside, Wanda seeks to explain.

Her husband is not someone who talks easily, she says. "I ask him to try, to let his feelings out; he really can't. He's trying to write them down. Sometimes, it's true, he has just … followed his feelings. And sometimes he has done things that are not in his own best interests, not at all, so as not to hurt or upset people. But he is a good man."

Both Breivik and Wanda are sure he will never be able to return to Norway. "Some people do feel I am guilty," he says. "I do have feelings of shame, disgrace. Damnation. Maybe … maybe I am to blame."

He has not kept any photographs of Anders, not even as a small boy, for a long time. He moved around a great deal with his job, of course. "But also," he says, "sometimes, when you have made a very serious mistake, you just want to forget it. Not be reminded."