As a reeling nation raced to gather information on the worst mass shooting in modern U.S. history Sunday, false reports and hoaxes masquerading as reliable information quickly rose to fill the void.

The false stories that followed the attack on a country music festival in Las Vegas brought into stark focus how easily systems meant to combat online lies can still be fooled.

The falsehoods crept into prominent Google search results and Facebook’s Crisis Response page — among them a post on anonymous message board 4chan that accused an innocent man of being a leftist militant and the mass murderer.

Police later named Stephen Paddock, 64, who was found dead in a hotel room at the Mandalay Bay Resort and Casino, as the lone gunman responsible for shooting hundreds of concertgoers.

Google and Facebook removed stories and links naming the wrong man, but more false reports continued to be shared on social media.

Fake victims. Fake shooters. Fake grief. Fake facts.

Internet rumors and hoaxes have become a fixture of high-profile tragedies and disasters, but by allowing them, at least for a while, to stand alongside legitimate news stories, Facebook and Google granted them implicit credibility.

The companies on Monday blamed the results on algorithmic miscalculations — the result of relying on machines’ artificial intelligence.

“Our Global Security Operations Center spotted these posts this morning and we have removed them,” a Facebook spokesman wrote in an email. “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.”

Facebook eliminated the jobs of human moderators dedicated to screening trending news stories last year. It has recently hired thousands of people to moderate video content and political ads.

Google said the 4chan allegations did not appear in search results when users typed general inquiries about the Las Vegas shooting but, rather, showed up in its “Top Stories” section when users searched the wrongly accused man’s name.

Because the misidentified man had minimal search results associated with his name prior to the 4chan accusations, Google’s algorithms took the sudden uptick in interest as a breaking story, triggering Google’s “Top Stories” feature, which uses a mix of verified news stories and Internet posts.

“Unfortunately, early this morning we were briefly surfacing an inaccurate 4chan website in our search results for a small number of queries,” a Google spokeswoman wrote in an emailed statement. “Within hours, the 4chan story was algorithmically replaced by relevant results. This should not have appeared for any queries, and we’ll continue to make improvements to prevent this from happening in the future.”

On Facebook and Twitter, where tweets claiming to be urgent messages about missing loved ones ran rampant, fake information can be even more difficult to discern as the posts mirror legitimate cries for help.

A Twitter spokeswoman on Monday pointed to a June blog post from Colin Crowell, Twitter’s vice president of public policy, government and philanthropy, when asked about the false tweets cropping up on the social network. In the post, Crowell passed the buck to “journalists, experts and engaged citizens,” who, he said, should be the ones to correct falsities.

“Twitter’s open and real-time nature is a powerful antidote to the spreading of all types of false information. This is important because we cannot distinguish whether every single tweet from every person is truthful or not,” Crowell wrote. “We, as a company, should not be the arbiter of truth.”

The Twitter spokeswoman added that the company was “reviewing and removing content that violates our rules — both proactively and through (user) reports.”

Facebook, Google, Twitter and other tech giants have for nearly a year resisted taking responsibility for the proliferation of false information online, in part, experts have speculated, to dodge regulation and government oversight.

Tech companies have instead announced moves to cut the flow of advertising dollars to fake-news websites, partnered with journalists and professional fact-checkers to check on trending stories and topics, and created features meant to help users discern what information is true and what is not.

It remains unclear what impact any of these efforts have had.

In April, Google released a report that stated 0.25 percent of its daily traffic returns have offensive or “clearly misleading content, which is not what people are looking for.”

Marissa Lang is a San Francisco Chronicle staff writer. Email: mlang@sfchronicle.com Twitter: @Marissa_Jae