The catalogue entry from the museum that holds the painting names the thief, his trade and the year — and stops there. That is a good source that cannot answer the question on its own.
The note below shows how the day-level date was reached, and why the arrest two years later is written into the question as a neutralised decoy.
The question as published
An Italian glazier who had worked for a Paris museum walked out of it early one Monday morning wearing a staff smock, taking a painting that was only recovered after his arrest in December 1913. On what date did he take it?
- Answer
- 21 August 1911
- Confidence
- Medium-high — day-level date from two corroborating records, year confirmed by the museum
Sources opened and read
Portrait de Lisa Gherardini, épouse de Francesco del Giocondo
Musée du Louvre (collections catalogue) • Museum catalogue record • Primary
The holding institution names the thief, his job and the year — but not the day, so it corroborates without leaking the answer.
Open sourceVincenzo Peruggia
Wikipedia • Reference database • Secondary
Gives the day, the weekday and the arrest date, all attributed to the interrogation record.
Open sourceMona Lisa
Wikipedia • Reference database • Secondary
A second, separately maintained article giving the same date, checked directly rather than assumed.
Open source
Searches that were run
when was the Mona Lisa stolen
Google • 4 September 2026 • Answer exposed: Yes
Exposes 21 August 1911 in the snippet.
Louvre glazier Italian arrested Florence 1913 painting returned
Google • 4 September 2026 • Answer exposed: No
Returns material on the recovery; the 1911 date appears only after reading a record.
musée du Louvre catalogue vol 1911 Peruggia
Google • 4 September 2026 • Answer exposed: No
Reaches the Louvre catalogue record used as E01, which gives the year but not the day.
"12 December 1913" Mona Lisa arrest date
Google • 4 September 2026 • Answer exposed: No
Returns the recovery date, the most common wrong answer.
What the investigation showed
What made the question difficult
Two dates circulate — the 1911 theft and the December 1913 arrest — and the museum's own catalogue gives only the year.
Which searches failed
The Louvre catalogue search reached an authoritative record that could not answer the question on its own.
Which source proved decisive
The account of the Florence interrogation, because it fixes the weekday and the hour as well as the date.
How ambiguity was removed
Naming the arrest date inside the question removes the most common wrong answer before the solver starts.
What an AI browsing agent might get wrong
Returning December 1913, or giving 1911 without a day because it stopped at the museum record.
Remaining limitations
The day-level evidence is encyclopaedic rather than archival; a digitised French police or press record from August 1911 would strengthen it.
Recommended future improvements
Add a contemporary Paris newspaper page from the days after the theft, retrieved from a national library archive.
Read the full case study
The complete record — clue matrix, evidence cards, validation log and the structured JSON output — is published on the case study page.
Open Cultural Heritage Benchmark