BRUSSELS — Serbia's ambassador to NATO was chatting and joking with colleagues in a multi-story parking garage at Brussels Airport when he suddenly strolled to a barrier, climbed over and flung himself to the ground below, a diplomat said.

By the time his shocked colleagues reached him, Branislav Milinkovic was dead.

His motives are a mystery. Three diplomats who knew Milinkovic said he did not appear distraught in the hours leading up to his death Tuesday night. He seemed to be going about his regular business, they said, 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.