A chilling 19th-century postcard written by someone claiming to be "Jack the Ripper" has resurfaced in the U.K., puzzling experts.

Once used by police in an attempt to catch the serial killer, the postcard will be offered for sale by Grand Auctions on April 30.

The card, which measures 2.75 inches by 4.75 inches, was received at Ealing Police Station in West London on Oct. 29 1888. It arrived when the city was reeling from a spate of vicious killings attributed to Jack the Ripper, who was never caught.

The killer is thought to have claimed the lives of least five young women in the Whitechapel area of London between August and November 1888. Mary Jane Kelly, the last of the so-called “canonical five” victims, was savagely murdered on Nov. 9, 1888 — just days after the postcard was delivered.

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“Beware there is [sic] two women I want here they are bastards, and I mean to have them my knife is still in good order it is a students [sic] knife and I hope you liked the kidney. I am Jack the Ripper,” the postcard states.

The kidney appears to be a reference to the famous “From Hell” letter sent in October 1888 by someone claiming to be Jack the Ripper. The letter, which was received by George Lusk, head of the Whitechapel Vigilance Committee, was sent with half a human kidney. Both items have since been lost.

The "I am Jack the Ripper" card has been checked by former police officer Stewart Evans, author of “Jack the Ripper: Letters from hell,” who says that the artifact is from the time of the killings.

Mystery, however, continues to swirl around the postcard’s author. "We are in the fortunate position of nobody being able to say the card was not written by Jack, but equally we cannot prove it was. It's one of those conundrums, you will never know,” said Jonathan Riley, a 19th-century history expert at Grand Auctions, in comments obtained by SWNS.

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The macabre note was held by police until the 1960s. “It was given to the vendor's husband, a Police Constable and member of the Metropolitan Police Force as a memento for his retirement from the force in 1966,” explains Grand Auctions on its website.

The postcard has a pre-sale estimate of $835 to $1,253.

More than 100 years after the brutal killings, Jack the Ripper continues to be a source of fascination. Earlier this year, a forensic linguist shed new light on letters supposedly written by the murderer during the killing spree that sent shockwaves through Victorian London.

Last year, in a separate project, researchers claimed to have proved the authenticity of a controversial Victorian diary that was supposedly written by the infamous murderer.

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Separately in 2017, another team of researchers looking to identify the last known victim of Jack the Ripper revealed that they have hit a low-tech roadblock that will likely prevent them from unleashing their DNA testing technology on any potential remains.

Experts from the U.K.’s University of Leicester that identified the remains of King Richard III had embarked on the project to identify Mary Jane Kelly.

However, finding Kelly’s actual remains poses a massive challenge. The grave marker in St. Patrick’s Catholic Cemetery in Leytonstone, London, where Kelly is assumed to be buried, likely has little relevance to her actual grave.

Follow James Rogers on Twitter @jamesjrogers