[Warren]

I have gone through my files and found this from Point de Vue, 14 March 1991 (paraphrased):

An Inexplicable Death

Prince Franz Josef Wenceslas of Liechtenstein (known as Wenzel) was found dead in his bedroom in Vaduz on 28 February 1991. An announcement from the Liechtenstein family Press Office stated that Prince Hans Adam "did not understand."

The circumstances of his death are most mysterious, and there are many rumours. Found in his bedroom were two chronometers (or timepieces) and an anesthesia mask. He may have been conducting a "medical experiment", or may have been experiencing a deliberate "temporary cessation of breathing", or he may have had a heart attack. His sudden death caused profound shock among his family and friends.

In 1982 Prince Wenzel entered the Military Academy at Sandhurst, and a year later was a lieutenant in the Grenadier Guards in London. He then studied medicine at Freiburg, and at the time of his death was an intern at a hospital in Rorschach in Switzerland, while still living in Vaduz. He was 28 years old and single.

Prince Wenzel's funeral was held in Vaduz on 6 March in the presence of the Princes of Liechtenstein, the Prince of the Asturias, Prince Guillaume of Luxembourg, Archduchess Walburga of Austria (using the title Duchess of Bar), Archduchess Regina of Austria [wife of the Archduke Otto, Head of the Imperial House], and representatives of the families of Bavaria, Baden, Württemberg, Hesse, Yugoslavia, and others. Photographs in the article show Prince and Princess Max of Bavaria [now Duke and Duchess in Bavaria], Erich, Erbgraf (Hereditary Count) von Waldburg zu Zeil und Trauchburg, and Prince Nicolas of Liechtenstein with his wife neé Princess Margaretha of Luxembourg and their daughter Princess Annunciata.

Prince Wenzel was interred in the family crypt where he lies with his parents Prince Franz Josef II and Princess Gina.

my ps.. I assume there would have been an autopsy carried out in Vaduz, so the cause of death would be known. It may be significant that it has not been made public.