Cohen was brought before Thames magistrates on 7 December 1888 as a lunatic wandering at large. He was found rambling in the street, speaking little but Yiddish. The magistrate sent him to the Whitechapel workhouse infirmary, where he was given the name of David Cohen, which was supposedly used as a John Doe for East End Jews without known identity, address or next of kin. He was born in 1865, his age was given as 23, his description as brown hair, brown eyes and a beard, and his address as 86 Leman Street. At the workhouse he was reported as violent, noisy and difficult to manage, he also attacked the other inmates. On 21 December he was transferred to Colney Hatch lunatic asylum, where his occupation was given as a tailor. Colney Hatch asylum was opened in 1851, and in 1858 had 1,293 patients, it closed in 1993, and has since been converted into luxury flats. At Colney Hatch his violent behaviour continued and he had to be separated from the other patients, he refused food and had to be fed by a tube, and was described as dirty, restless, aggressive and destructive. On 28 December he became physically ill, and over the next nine months his condition deteriorated. He died on 15 October 1889 from exhaustion of mania and pulmonary phthisis.
Chief Inspector Donald Swanson's pencil notes written in his copy of Sir Robert Anderson's book reveal that he suspected an insane Jew of the murders. He goes on to name the man as Kosminski, and says, 'The man died soon after being sent to Colney Hatch, having first been sent to the Whitechapel workhouse infirmary'. It has been suggested that as Kosminski did not die until 1919, Swanson's suspect in reality may have been Cohen, who fit's the criteria of having died soon after being sent to Colney Hatch. The drawback to this theory however, and of Cohen being the Ripper, is that Swanson unequivocally names Anderson's suspect as Kosminski, as does Sir Melville Macnaghten in his memoranda. Also, it is noted that the CID were still on Ripper alert after Cohen's death. Swanson states, 'After the suspect had been identified at the seaside home'. As the first of the convalescent police seaside home's was not opened until March 1890, at 51 Clarendon Villas, West Brighton, this makes the suspect identified unlikely to have been Cohen, because by this time Cohen was already dead.
Nevertheless, David Cohen is the only insane Polish Jew committed to an asylum at the right time for the murders to cease, and the only registered lunatic- pauper admitted to Colney Hatch between 1888-1890, who fit's the extremely violent suspect described by Anderson, Macnaghten and Swanson. He also, in contrast to Aaron Kosminski, died shortly after the canonical Ripper murders ceased.