The official account: Crown Prince Rudolf and his mistress Mary Vetsera were found dead at the Mayerling hunting lodge on 30 January 1889. Murder-suicide. The Austro-Hungarian Empire suppressed the details for decades.
The diplomatic dispatch — found in the Austrian State Archives in a misfiled box, sealed since 1893 — is addressed to the British Foreign Office. Its final paragraph reads:
"The Prince did not act alone in his despair. He was assisted. The instrument of his assistance left Vienna the same evening on the 11:15 train to Paris. His name and the evidence are distributed across four locations known to the Prince's circle."
You have the dispatch. You have the four locations. Someone is trying to have it reclassified.
Start at the Hofburg Imperial Palace.