It was January 2001 and I was training students to teach English as a foreign language. The year before, when I'd arrived in Bangkok, the newspaper headlines were full of reports about Eric Rosser, a convicted paedophile from America. He was due to stand trial for child molestation, rape and pornography, but had jumped bail and escaped.

I'd like to say it was easy to spot something strange about Peter Hill when he showed up in my class. But it wasn't like that at all. He was quiet at first; he'd sit alone, reading the Bangkok Post, and never went outside during breaks like my other pupils. But once he got into the swing of class, everyone liked him. He was personable, an average-looking guy in his 40s. He was clean, neat and did his homework on time.

On the first day of class, I asked my 10 students to pair up, interview each other and introduce their partner. Peter's partner told us that Peter was a musician. He wanted to help Thai people, and the best way to do that was to teach English to children.

Next morning, I sat next to Peter during break to chat. He said he was a British citizen. I asked why he didn't have a British accent and he said he'd moved to the US aged five. I asked if he had a green card and he said he didn't need one. I knew that he did, but I changed the subject by asking the class to go upstairs.

Soon after, I asked the trainees what classes they wanted to teach. I could always guess what they'd choose: men preferred adult conversation and business classes; women enjoyed teaching children. I was right about all the trainees, except Peter - he wanted to teach 10- to 12-year-old children.

At this point, I felt uneasy, but nothing more - until one Sunday evening a few weeks later. I was watching the TV show America's Most Wanted, and halfway through was the story of Eric Rosser, one of the FBI's 10 Most Wanted. There he was, on television. I was in shock; I couldn't believe it. This man was in my classroom every day, hoping to get a job teaching young children. Worse still, I knew I couldn't tell anyone. I had to act on my own. If "Peter" thought anyone knew who he really was, he would run.

I had to gather more information. I didn't want to go straight to the FBI unless I was sure I had the right man. I couldn't bear the idea of making a mistake and ruining someone's life.

All my life, I'd been a detective fan and read so many whodunits, so I spent a long time thinking about how I'd get the right information. I told the trainees I needed copies of their passports for my files. They all brought them in, except Peter, who had only a photocopy. His British passport showed that his birthday was July 8, but he'd told me it was January 17. The FBI's wanted poster had the same birthday: January 17 1958. I started a special file on him. I even brought my camera and told my class that I kept photos of each group I trained. I got his fingerprints, a tape of his voice and handwriting samples.

Then I called the FBI. We arranged to meet and they asked me to bring everything I had. When I showed Agent Robson the passport copy, she gasped, "It's him. It's him." She went to see Agent Cahill, who was also on the case. He called me later and asked about Peter. "What does he usually wear? What time does he usually arrive at the college? What direction does he come from?"

Rosser arrived at 9.45am the next morning. I knew the police would turn up any minute and I was nervous. How would he react? Would he try to run away?

Soon enough, six policemen came, along with the FBI. As they took him away, he turned and shouted, "It's all lies! I'm really a good person!" I'll remember those words for the rest of my life. Especially the way he looked. He was shouting but pleading at the same time. It was spooky. I felt sorry for him in a way - there were so many mixed emotions I still find difficult to explain.

Of course, I never saw him again. A month later I found out that Rosser had been convicted of all the charges against him in Thailand and the US. He'll be out in 2018, something I try not to think about too much.

The police told me that Rosser thought his Thai wife had shopped him to the FBI. I think he probably knows by now that it wasn't. I just wonder if he knows it was me.

· Do you have an experience to share? Email: experience@theguardian.com