The prisoner had been arrested in consequence of the discovery, in his room, by Detective Inspector Japp ”a most brilliant officer ”of the identical phial of strychnine which had been sold at the village chemist's to the supposed Mr. Inglethorp on the day before the murder. 
The strychnine was used to assassinate the Duke.
mnli hypothesis: The strychnine was used to assassinate the Duke. premise: The prisoner had been arrested in consequence of the discovery, in his room, by Detective Inspector Japp ”a most brilliant officer ”of the identical phial of strychnine which had been sold at the village chemist's to the supposed Mr. Inglethorp on the day before the murder. 