Police on Monday remained baffled as to why 64-year-old Stephen Paddock, the man they believe carried out the deadliest mass shooting in United States history, opened fire from the 32nd floor of the Mandalay Bay Hotel in Las Vegas Sunday night. But a widely circulated photo of Paddock, who reportedly killed himself as police closed in on his hotel room, contains a so-far unexplained detail — the number “13” apparently tattooed on Paddock’s neck.

Police have yet to comment on whether the number 13 tattoo can shed any light on Paddock’s motives for carrying out the shooting which killed at least 50 and injured hundreds. In fact, why the man who was described by family members as living quietly in retirement, according to the Las Vegas Review-Journal, would have suddenly snapped and opened fire on a crowd of 22,000 gathered at a country music concert below remains a mystery as of Monday morning.

However, law enforcement officials have revealed that Paddock made “several large gambling transactions in recent weeks,” according to an NBC News report. The “transactions” — not identified by police as either winnings or losses — were said to be in the five-figure dollar range, but whether they were somehow connected to the mass shooting remained unclear.

“We have absolutely no idea how in the world Steve did this. Absolutely no concept,” one relative of Paddock’s told the Washington Post. “There was nothing secret or strange about him.”

LAS VEGAS SHOOTING: Gunman Stephen Paddock Kills 50 People WHAT WE KNOW >> https://t.co/BVj2eUDNLk pic.twitter.com/nWilV8KnY9 — Breaking911 (@Breaking911) October 2, 2017

Police also said that nothing they had yet uncovered in Paddock’s background indicated that he had violent or criminal tendencies, according to the Post report, and other media accounts.

What, then, is the meaning of the number 13 — which can be clearly seen in the photo identified as Paddock published widely in the media and online Monday morning? The number which appears to be a crude tattoo, but may also be handwritten with a pen such as a felt-tip marker, can be seen directly above the collar of a white undershirt worn by Paddock in the photo, on the front of the man’s neck.

The photo was reportedly taken from the Facebook account of Marilou Danley, believed to be Paddock’s live-in female companion. No Facebook account for Paddock himself has been discovered and according to media reports, he did not have one.

An earlier photo of a younger Paddock, released by his brother Eric Paddock, shows no evidence of the “13” neck tattoo.

Eric Paddock (left) shared this picture of his brother Stephen Paddock (right). Says he feels like an astroid has hit his family. pic.twitter.com/3E7e0znC8r — Mark Lehman (@MarkLehman6) October 2, 2017

A dormant Twitter account attributed to a “Stephen Paddock” and started in 2010 — but containing no tweets — includes photos of a man who bears some resemblance to the Paddock in the widely circulated photo, but whether that man is the same Stephen Paddock remains uncertain. The man in the Twitter photos also displays no evidence of the crude “13” tattoo, but does visibly bear a more elaborate “Superman” insignia tattoo on his upper left arm, and another less legible tattoo on his upper right.

The number 13 is considered to have mystical or supernatural significance in traditions ranging from Norse mythology to the Christian New Testament. But while the number is generally considered “unlucky,” according to the blog Secret Ink Tattoo 13 is often considered to be the reverse by tattoo artists and enthusiasts — a “lucky number.”

“This is due perhaps to some tattooists and tattoo collectors considering themselves as separated from everyday society,” the blog’s author wrote in a 2013 post.

But the 13 tattoo — if indeed it is a tattoo — seen on Paddock’s neck in the photo appears too crudely drawn to be the work of a professional tattoo artist. The number is also a commonly appearing prison tattoo, generally signifying membership in the notorious El Salvador-based criminal gang Macha Salvatrucha, better known as “MS-13.”

The number 13 is also reportedly a symbol used by the white supremacist prison gang “Aryan Circle,” with the numbers “1” and “3” substituting for the “A” and “C” which are both the first and third letters of the alphabet and the initials of “Aryan Circle.”

Police say that Paddock has no criminal history and therefore would not have served a prison term, and at least as of Monday morning, is not believed to be affiliated with any criminal organizations.

The possibility also remains that the marking on Paddock’s neck is not the number 13 at all, but some other, as yet unidentified symbol.

BREAKING: #ISIS claimed #LasVegas attack, reporting through ‘Amaq that executor is one of its “soldiers” & he converted to Islam months ago pic.twitter.com/4E85vEIUzH — SITE Intel Group (@siteintelgroup) October 2, 2017

The international terrorist group ISIS claimed responsibility for the Las Vegas attack in a posting on its propaganda outlet Amaq. The ISIS message claims that the shooter “converted to Islam a few months ago,” but U.S. law enforcement officials said that there was no evidence to connect Paddock to any international terrorist group, and the ISIS posting cited no evidence or support for its claims.

[Featured Image by David Becker/Getty Images]