I mentioned a recent FT interview with Frits Veerman which is behind a paywall. But I have since found an interview that Transparency International conducted with Veerman in 2018:

Veerman only had his suspicions. He knew he needed solid evidence, so he started correspondence with Khan. “Abdul and I shared a hobby: astronomy. I started to write him letters about that until I had the proof about what he was doing exactly. At one point he asked if I could share technical information about an ultracentrifuge with him by mail. There I had him, there I had proof that he was spying.” When Veerman showed the letters to the FDO management, they summoned him to destroy them. But he didn’t. He provided a copy to management, and archived the original letter. To this day, the letters are neatly and securely stored in his safe.