The culprit is Masuyo Mamiya. She is in reality a maid named Mutsumi Kishikawa, who killed the grand mistress and took over her identity to search the castle for the treasure.
People
Katsuo Tabata
41 years old
Mamiya family gardener
Mitsuru Mamiya
49 years old
Husband of the eldest Mamiya daughter (Married into the family)
The title for each chapter are worded as the following in Case Closed:
File 201 - Another Victim
File 203 - The Ivory Tower
File 204 - The First Entry
File 205 - A Locked Room in the Sky
File 207 - A Secret Hidden in the Heart
File 208 - The Sakuradamon Incident
File 209 - The Inspector's Deduction
File 210 - An Unforeseen Enemy
File 211 - A Perfect Day for Tokyo
Cover in other countries
China
Finland
France
Germany
Hong Kong
Indonesia
Italy
Korea
Malaysia (Chinese)
Malaysia (Malay)
Norway
Rep. of China (Taiwan)
Spain (Spanish, Volumen 2)
Spain (Spanish, Volumen 2)
Sweden
United States
Vietnam
Trivial
In the Danish version Mutsumi said that she killed the grand mistress and took over her identity, because the old lady wanted to fire her for searching for the treasure.