Returning to England, he married a Quaker lady as his second wife. He confessed to the murder of his mistress, Sarah Hart, by prussic acid, his motive being a dread of their relations becoming known.
Between six and seven o'clock one morning in 1845 a woman named Sarah Hart was found dead in her home at Salt Hill, and a man had been observed to leave her house some time before. The police knew that she was visited from time to time by a Mr John Tawell, from Berkhamsted, where he was much respected, and on inquiring and arriving at Slough, they found that a person answering his description had booked by a slow train for London, and entered a first-class carriage.
The police telegraphed at once to Paddington Station, giving the particulars, and desiring his capture. 'He is in the garb of a Quaker,' ran the message, 'with a brown coat on, which reaches nearly to his feet.' There was no 'Q' in the alphabet of the five-needle instrument, and the clerk at Slough began to spell the word 'Quaker' with a 'kwa'; but when he had got so far he was interrupted by the clerk at Paddington, who asked him to 'repent.' The repetition fared no better, until a boy at Paddington suggested that Slough should be allowed to finish the word. 'Kwaker' was understood, and as soon as Tawell stepped out on the platform at Paddington he was 'shadowed' by a detective, who followed him into a New Road omnibus, and later arrested him in a coffee tavern. Tawell was tried for the murder of the woman, and revelations were made as to his character.
Tawell was hanged in public at Aylesbury, and the notoriety of the case brought the telegraph into repute. Its advantages as a rapid means of conveying intelligence and detecting criminals had been signally demonstrated, and it was soon adopted on a more extensive scale. It was also, as far as we know, the first homicide case where the criminal attempted to flee the scene of the crime by a railway train.