Nicknamed "the Paper Man" due to his height and slim build, Sindelar spent his entire professional career with FK Austria Vienna, joining them in 1924. He debuted for the Austrian national team in September 1926, scoring a goal in a 2-1 win over Czechoslovakia. He soon became an integral member of the Wunderteam, winning the 1932 Central European International Cup, taking silver in the 1936 Summer Olympics, and finishing fourth at the 1934 World Cup. In all, Sindelar made 43 official appearances for Austria, scoring 27 goals.
Following Austria's annexation by Germany in 1938, Sindelar declined to play for the new unified team despite repeated requests by German manager Sepp Herberger. Although he cited age as the basis for his refusal, others took it as a sign of protest against the Nazi regime. Thus, when he was found dead a few months later, conspiracy theories ran rampant. Some suggested that he committed suicide in order to avoid Nazi persecution, while others suggested that he and Castagnola had been murdered by the Nazis for harboring Jewish sympathies.
While one the two chimneys in Castagnola's apartment was known to be defective, making carbon monoxide poisoning the most likely explanation for their deaths, one of Sindelar's friends gave a 2005 interview suggesting otherwise. In 2005, a BBC documentary crew spoke with Egon Ulbrich, a longtime friend of Sindelar's, who claimed that a Vienna official had been bribed to record the deaths as accidental. According to Ulbrich, Nazi rules prevented state funerals for victims or murder or suicide, so the declaration of Sindelar's death as accidental ensured that he could be buried with full honors.
[For more on the deaths of Sindelar and Castagnola, see the excellent Soccernet story by Uli Hesse-Lichtenberger here.]