In April, 1896, HH Holmes published a "confession" in which he claimed to have killed twenty-seven people. Actually, there were two, and possibly three, confessions written out from his prison cell.
The main confession was written for the Philadelphia Inquirer. Consisting of about 10,000 words, he claimed early on that he had killed twenty seven people, and proceeded to detail each murder. However, he may have left a couple out; he only actually mentions 24 or 25 of them (including a couple of unborn babies). A few of the people he confessed to killing, though, were actually still alive, and others appeared never to have existed in the first place. One (Gertrude Conner) was found to have died of neuralgia of the heart several weeks after leaving Holmes' employ, and was presumably not a victim (though a slow acting poison isn't out of the question). A couple more were un-named. The police claimed at the time that they only suspected Holmes of killing nine or ten people, and that the confession didn't give them a single reliable name to add to the list. They may have been wrong; there was one girl, Anna Betts, whose death certificate may back Holmes' story up. A few others were unnamed and couldn't be confirmed or denied.
The other confession was written for the New York Morning Journal. Like the Inquirer, it ran a hand-written statement from Holmes saying that this would be his only confession. It was largely the same as the Inquirer, but with stories of six or seven victims omitted, one story expanded, and few minor word changes (most likely, Holmes wrote out the first and then copied it down, making minor changes as he went). This was the version that was published in most of the regional papers, including the Chicago Daily Inter-Ocean.
The day before these two were published, another confession appeared in the Philadelphia North American. The paper claimed to have seen advance copies of the confession that was to be published the next day, and a few things it lend credibility to the idea that they had, in fact, seen a copy. However, most of the text of their version, including a story about mutilation his son and the famous "I was born with the devil in me" section, was not present in either of the confessions published the next day, and the Inquirer included a hand-written bit from Holmes saying that other confessions printed were "false." So, where this one came from is sort of a mystery. It's possible that they saw an earlier draft, and it's possible that Holmes sold them another version under the table (this is just the sort of thing he WOULD have done). The writing style of this version is a bit more dramatic, though, and my guess is that they saw or heard just a bit of the confession and wrote up a bunch of extra stuff themselves. But papers of the day seemed to believe them; many papers, including the Tribune, published excerpts.
A century later, we're really no further on the case than the police were. We still can't really NAME more than a dozen or so victims. We suspect the number is much higher, but we don't have much more information about who those other victims might be! I'll be running a Devil in the White City tour tonight with Ursula Bielski and Jeff Mudgett, Holmes' great great grandson. If you want to know more about what was in the confession, there's a new ebook which includes the full texts (with notes on differences between versions) and contains highly detailed analysis of how the confession lines up with what we know of the truth