In Columbia I had filmed with the students, dreaming of a better world, being together in a way that seemed daring and right. I was warned to leave America – they were after my film, which I’d prudently hidden in a fridge in a friend’s flat. Suddenly, film and reality were becoming indistinguishable. Arriving back in London, the headlines greeted me: Bobby Kennedy shot dead. I had filmed a whole day with him, three weeks before. Had I killed him?