It was a Sunday, maybe two in the afternoon, in November, when Michael Moore witnessed his first murder. His mother was vacuuming and he was sitting on the floor, watching TV. During a report about President Kennedy's assassin Lee Harvey Oswald, he saw nightclub owner Jack Ruby put a gun to Osward's ribs and shoot him.

Another killing made an equal impression as a youngster. "I remember coming out of mass, and one of the dads had gone to the car to warm it up," he recalls. "His radio was on and the announcer said they had shot Martin Luther King. A cheer went up from people coming out of church. When you're 13 years old, this stuff burns in your head," says Moore.

Nowadays he's gunning for trouble himself in the documentary film Bowling For Columbine. America's gun culture is his target in the wake of the Columbine High School massacre in Colorado, and the shooting of a six-year-old girl by a young classmate in his hometown of Flint.

In the flesh, Moore's a big bear of a man, complete with his trademark baseball cap. This is the man who took on General Motors in his 1989 film Roger and Me, and sports shoe giant Nike in the 1997 documentary The Big One.

Bowling For Columbine begins as he opens an account in a bank offering customers a free gun as an incentive. Later, he leaves veteran actor and National Rifle Association chairman Charlton Heston lost for words after confronting him over his support of US gun laws.

More incredibly, he takes two Columbine victims, both with bullets still embedded in their bodies, to the headquarters of K-Mart to ask for a refund from the supermarket chain that sold the ammunition. Even he's surprised when K-Mart announces it will stop selling ammunition. "Columbine was the trigger but this film has been percolating in my head for a long time," explains Moore. "To do something about the American thirst for violence and why we see, even use, violence as a means to an end. You can't help but think about it if you grew up in America."

The TV images in the aftermath of the Columbine massacre have stayed with him. "I remember the kids running out with their hands above their heads, because potentially they were all guilty. The police lined them up. These were not monsters, they were normal kids and it could really happen to anybody. That's too frightening," he says.

His approach to making documentaries is scattershot. "I just start shooting and whatever happens, happens. The best is a happy accident," he explains.

He travelled to Canada, which has seven million guns in ten million households, expecting to find a situation similar to the US. In fact, the gun murder rate is low. They have a lot of guns, but don't shoot each other. Moore attributes that to the climate of fear in America, one that breeds a society that feels the need to own guns.

The K-Mart outcome shows he can make a difference, but doesn't fool himself that he has all the solutions. "In part I want the audience to leave the cinema and do something, or at least talk and think about it," he says.

He's not only preaching to the converted. More people are going to see documentaries in movie theatres in the US, with Bowling With Columbine reaching a much wider audience than expected for this type of film. The film opening at a time when a sniper was headline news must have made audiences think long and hard about their gun culture. "If ten per cent leave the theatre thinking about what I have said, I've scored a huge victory. If five per cent do something, maybe something will happen. There are millions and millions of people who will say some of the things I say. I'm not alone. You can see that from how well the movie is doing. It's an indication of something going on out there."

Moore, currently appearing in his one-man stage show in London, admits he gets nervous-"my stomach is in a thousand knots" - before a big confrontation. While filming Bowling For Columbine, one interviewee took him into his bedroom to show him the loaded gun he kept under his pillow. Then he put the gun to his head and pulled back the trigger as Moore looked on in amazement.

He believes journalists should be asking the questions he does, and that the media should be embarrassed he's the one making life difficult for corporate America.

While he doesn't think violence in movies contributes to violence in society, he does believe the constant diet of bad news presented as reality does affect people. "News programmes say, 'this is what happened today'. But it's not, it's six stories they have picked out to frighten people."

An obvious question is why do people talk to him when they know his reputation? "They want to be on TV," is his simple answer.

"I would never talk to me. I don't know why they do. I give people a fair chance. I don't go there and do things which maybe other people do just to make them look bad. I just turn on the camera and let them speak."

* Bowling For Columbine (15) opens at Newcastle Tyneside Cinema on November 29 and at York City Screen on December 13.