Conversation so far:
What was the Mirror challenge?
challenged Houdini to escape from special handcuffs that it claimed had taken Nathaniel Hart.
Who challenged Houdini?
London Daily Mirror newspaper.
Who was Nathaniel Hart?
a locksmith from Birmingham.
What was special about the handcuffs?
five years to make.
When did he complete the challenge?
March 17.
Where did he do it?
London's Hippodrome theater.
How much money did he earn for his performance?
CANNOTANSWER.
What recognition did he get for performing the challenge?
After Houdini's death, his friend Martin Beck was quoted in Will Goldston's book, Sensational Tales of Mystery Men, as admitting that Houdini was bested that day.
How was he bested?
found evidence (notably in the custom design of the handcuffs) that the Mirror challenge may have been arranged by Houdini.

Question: What evidence did they find?
Self-contained rewrite: What evidence did investigators find about how Harry Houdini did the mirror challenge?