As History Today explains, Émile Zola died from what was ruled at that time to be carbon monoxide poisoning from a coal fire lit in his bedroom that night. Zola's wife, Alexandrine, was also sickened but recovered. Prior to that point, Zola's outspoken position on the Dreyfus Affair had drawn criticism from some corners of French society who contended Dreyfus (above, in prison) was guilty, and particularly from right-wing French nationalists. He's said to have received threatening letters from so-called anti-Dreyfusards, according to a 2002 Los Angeles Times report. For this reason, when Zola's mistress learned that Zola had died she immediately assumed it was murder, as History Today goes on to note.
Regardless of those suspicions, Zola's official cause of death was ruled to be accidental carbon monoxide poisoning. Some 50,000 people attended his funeral, including many high-ranking government officials and Dreyfus himself. Rumors that Zola was murdered, however, quickly spread, so much so that an inquest was ordered, and the house in which he died and in which his wife was sickened was tested. Fires were lit to test for carbon monoxide, and guinea pigs were left in the same room overnight, among other experiments.
