By Glenn Chapman
It almost looked like a first-person shooter video game: a handgun in a man's grasp comes into view. A woman appears in the gunman's sights. Shots are fired.
But this was no game. It was the chilling video by a killer of him stalking his prey - two television journalists live on-air for a station in the US state of Virginia that fired him for erratic behaviour in 2013.
A small audience will have witnessed the attack live on TV. But internet users, even if only for a brief time, had a first-person view of his horrific crime after the killer fired off tweets while on the run and then posted his sickening video online.
"This seems like we're turning a new page in the internet era," Endpoint Technologies Associates analyst Roger Kay said.
"It's definitely a new frontier. Except for ISIS, really nobody has done this."
Islamic State extremists have used social media to share videos of beheadings to advance their cause through terror, but the shooter's video marked the first time "a freelancer" operating for personal reasons used the shocking sharing tactic, according to Kay.
WDBJ reporter Alison Parker and cameraman Adam Ward were shot dead while they were conducting a live interview in the town of Moneta, outside Roanoke in southern Virginia.
Ward's footage for the station ends with a terrifying view of the shooter standing over him, his gun pointed down at the victim. It shocked people back in the newsroom, including Ward's fiancee, and others watching.
The station then hastily cut away to a startled anchorwoman back in the studio.
But hours after the shooting, the 56-second video clip apparently taken by gunman Vester Lee Flanagan, also known as Bryce Williams, went viral, before it was removed from social media sites.
Tweets and the video were posted to a Twitter account with the handle @Bryce_Williams7 and also on Facebook. Flanagan died from a self-inflicted gunshot wound after fleeing the scene.
His video, seen by AFP, showed the shooter walk along a wooden deck to where Parker, Ward and the woman being interviewed were standing. All appeared not to notice the person pause nearby and take aim slowly and methodically with a pistol.
The video did not show who was holding the camera, but it appears the gunman was operating it.
One of the tweets read: "I filmed the shooting see Facebook." The video clip followed.
Other tweets, evidently posted while Flanagan was on the run, referred to "racist comments" allegedly made by Parker.
"They hired her after that???" it said.
The first-person video was shared at Twitter, Facebook and YouTube, while imagery from the television broadcast spread online in a disturbing example of the speed and power of social media.
Auto-play features that instantly launch videos at Facebook or Twitter resulted in some viewers being shown startling imagery before having time to consider whether they wanted to see it.
A Facebook page where the first-person video had been uploaded and its associated profile were removed for violating terms of service that include a ban on "celebrating any crimes you've committed", according to the social network.
The video posted to Twitter was removed on the grounds it violated its media policies. Twitter stuck to its practice of refusing to discuss details of individual accounts.
Copies of the first-person shooting video posted at YouTube were similarly quickly taken down, but at least one copy was still visible late on Wednesday with a search as simple as "Virginia".
"I know it's a sad occurrence, but leave a Like so this can spread faster," a YouTube user who uploaded a copy of the shooter's video said in a chat forum beneath the post, before it too was removed.