As law enforcement and news organizations raced to piece together what happened during the worst mass shooting in modern U.S. history Sunday night in Las Vegas, web denizens less wedded to the truth rushed in to provide details of their own—which quickly went viral.

Links to the 4chan website that falsely identified the shooter and called him a leftist and Democratic supporter were showing up on the top of Google search results, according to tweets by Buzzfeed News reporter Ryan Broderick.

Also, apparently Google is putting 4chan threads in their top story unit now? So, the number one hit for his name is a /pol/ thread. pic.twitter.com/OYwW6pbWvy

— Ryan Broderick (@broderick) October 2, 2017

Conservative writer Joe Hoft pounced, publishing and then retracting an article about the misidentified man. Police later identified a different person, Stephen Paddock, as the shooter.

A few hours later, searches for the same name were showing articles debunking the 4chan post and cataloguing the trail of viral fake news after the shooting.

Once police identified Paddock, accounts on Twitter and Facebook began claiming he was part of the leftist group Antifa.

A spokeswoman for Google wasn't able to immediately provide a comment.

Separately, Facebook's "trending" list took viewers to a feed of articles, some of which appear to spread misinformation from sites like RT, formerly known as Russia Today. One story from RT.com, popular on Facebook, introduces a post by saying the "Las Vegas attack was carried out by a soldier of ISIS."

Lower in the post, RT's headline notes that ISIS involvement was a "claim" by the terror group. Even that claim has been widely dismissed. RT has been among the publications scrutinized for its role spreading disinformation, seemingly as a propaganda arm of the Kremlin, on Facebook during the presidential election.

"Our Global Security Operations Center spotted these posts this morning and we have removed them," a Facebook spokesman said in an e-mailed statement. "However, their removal was delayed, allowing them to be screen captured and circulated online. We are working to fix the issue that allowed this to happen in the first place and deeply regret the confusion this caused."

RT was not the only outlet getting high Facebook visibility for a post alleging an ISIS connection:

Right now, FB's trending topic page for the Las Vegas shooting features two (2) posts from a Russian propaganda outlet. pic.twitter.com/jDR1V0zzPy

— Kevin Roose (@kevinroose) October 2, 2017

On Monday, Facebook was expected to hand over to Congress 3,000 ads created by fake news purveyors apparently out of Russia in a seeming attempt to affect the election. Facebook disclosed the existence of the ads last month, and has been discussing its failings during the election more openly of late. Over the weekend, CEO Mark Zuckerberg asked for forgiveness in a Facebook post and called for unity following the Jewish day atonement of Yom Kippur.

The incidents highlight yet again how news and social-media algorithms designed to help surface the best information can fall short in the hours after a major incident, when few factual details are readily available because authorities have yet to confirm or release them. Millions of concerned people, some potentially with family members affected by the shooting, likely Googled or searched on Twitter and Facebook for scraps of information in the hours after the attack.

Last week, lawmakers rebuked Twitter, saying the company's presentation about Russia-linked accounts on its own site didn't dig deep enough.

-- Bloomberg News with additional contributions by Ad Age staff