Serbia's ambassador to Nato threw himself to his death from a car park moments after chatting and joking with colleagues.
Branislav Milinkovic was at Brussels Airport when he suddenly strolled to a barrier, climbed over and jumped over, one diplomat said.
His motives are a mystery. Three diplomats who knew him said he did not appear distraught in the hours leading up to his death on Tuesday night. He seemed to be going about his regular business, picking up an arriving delegation of six Serbian officials who were due to hold talks with Nato, the alliance that went to war with his country just 13 years ago.
A former author and activist opposed to the authoritarian regime of Serbia's former strongman Slobodan Milosevic, he was a respected diplomat and leading intellectual.
The diplomats said they knew of no circumstances - private or professional - that would have prompted him to take his own life.
One described the death, saying she had spoken to a member of the delegation who had witnessed the leap from about 30 ft up.
Serbia's Prime Minister Ivica Dacic said that "Belgian police are investigating, but it's obviously a suicide. It's hard to figure out the motives or causes."
The death cast a pall on the second day of a meeting of Nato foreign ministers. Officials said they were shocked by the news of the death of a popular and well-liked ambassador.
During the 1990s, Mr Milinkovic was active in the opposition to Milosevic. After Milosevic was ousted in 2000, he was appointed Serbia's ambassador to the Organisation for Security and Co-Operation in Europe, or OSCE, in Vienna. He was transferred to Nato as Serbia's special representative in 2004. Serbia is not a member of the military alliance, but Mr Milinkovic was named ambassador after Belgrade joined Nato's Partnership for Peace programme, which groups neutral states.
Mr Milinkovic is survived by his wife and 17-year-old son.