As authorities continue to try and assess the situation after the U.K. attack on Wednesday, Gizmodo reports that some Internet users suspect the plan may have been posted to the site 4chan the day before.

The area around the British Parliament in London was thrown into chaos Wednesday afternoon after an attack which is believed to have killed at least four people and injured 20, reports the New York Times.
As authorities continue to try and assess the situation, Gizmodo reports that some Internet users suspect the plan may have been posted to the site 4chan the day before. 
The post is said to have appeared on the site’s politically incorrect imageboard from a user ID that was not linked to any other messages. 
While the time of the attack was reportedly not mentioned, The Independent says the post indicated the exact location through Morse code which led to a URL containing the coordinates.
The unknown user also included a photo of two guns, though no other information about any kind of plan was given.
According to the New York Times, the attack occurred around 2:40 pm when “a large vehicle mowed down pedestrians on [the bridge], which leads to Parliament...before crashing into a railing.” 
A man who exited from the car with a knife is said to have killed a police officer with it before being shot dead, notes the BBC.
Cmdr. B. J. Harrington of the Metropolitan Police has stated at a news conference that “a full counterterrorism investigation is underway.”