The attack in London yesterday that saw four people killed and 29 injured as a terrorist mowed down pedestrians and stabbed a police officer may have been announced less than 24 hours earlier on imageboard website 4chan.

A user with the ID n1SoXPxb made the post at 3 pm BST on Tuesday. As noted by Gizmodo, it is the only post made using that ID across any 4chan board, and while the user is flagged as Danish, they may have been using a VPN to disguise their location.

The user posted an image on the site’s politically incorrect (/pol/) section showing two guns taken from an unrelated 2010 forum post with the phrase “/pol/ 21-3-2017” edited into the picture. They also included a series of dots and dashes that turned out to be Morse code for “gotohttp://pastebin.com/zluxdmza.”

Following the link to the Pastebin page shows a binary code, which, when ran through a binary-to-text translator, reveals a set of GPS co-ordinates positioned right next to Big Ben - just off Westminster Bridge where the attack took place.

Many 4Chan users say the post is real and some even fear the site may be shut down, while others are more skeptical – this wouldn’t be the first time a 4chan post seemingly warning of an attack turned out to be fake. But in this instance, there is compelling evidence to support its authenticity: the Pastebin page could not have been altered after the attack because it was a guest account, and 4chan users who responded to the post on Tuesday noted the code’s reference to Big Ben.

4chan owner Hiroyuki Nishimura told Gizmodo: “understand the situation. We are ready to respond for law enforcement agency.”