WASHINGTON — Four minutes into a video that was posted on Instagram last month, Justin Bieber leaned into the camera and adjusted the front of his black knit beanie. For some of his 130 million followers, it was a signal.

In the video, someone had posted a comment asking Mr. Bieber to touch his hat if he had been a victim of a child-trafficking ring known as PizzaGate. Thousands of comments were flooding in, and there was no evidence that Mr. Bieber had seen that message. But the pop star’s innocuous gesture set off a flurry of online activity, which highlighted the resurgence of one of social media’s early conspiracy theories.

Viewers quickly uploaded hundreds of videos online analyzing Mr. Bieber’s action. The videos were translated into Spanish, Portuguese and other languages, amassing millions of views. Fans then left thousands of comments on Mr. Bieber’s social media posts asking him if he was safe. Within days, searches for “Justin and PizzaGate” soared on Google, and the hashtag #savebieber started trending.

Four years ago, ahead of the 2016 presidential election, the baseless notion that Hillary Clinton and Democratic elites were running a child sex-trafficking ring out of a Washington pizzeria spread across the internet, illustrating how a crackpot idea with no truth to it could blossom on social media — and how dangerous it could be. In December 2016, a vigilante gunman showed up at the restaurant with an assault rifle and opened fire into a closet.

In the years afterward, Facebook, Twitter and YouTube managed to largely suppress PizzaGate. But now, just months before the next presidential election, the conspiracy theory is making a comeback on these platforms — and on new ones such as TikTok — underlining the limits of their efforts to stamp out dangerous speech online and how little has changed despite rising public frustration.

This time, PizzaGate is being fueled by a younger generation that is active on TikTok, which was in its infancy four years ago, as well as on other social media platforms. The conspiracy group QAnon is also promoting PizzaGate in private Facebook groups and creating easy-to-share memes on it.

Driven by these new elements, the theory has morphed. PizzaGate no longer focuses on Mrs. Clinton and has taken on less of a political bent. Its new targets and victims are a broader assortment of powerful businesspeople, politicians and celebrities, including Mr. Bieber, Bill Gates, Ellen DeGeneres, Oprah Winfrey and Chrissy Teigen, who are lumped together as part of the global elite. For groups like QAnon, PizzaGate has become a convenient way to foment discontent.

The theory has also gone global. While it previously found traction mainly in the United States, videos and posts about it have racked up millions of views in Italy, Brazil and Turkey.

“PizzaGate never went away because it encompasses very potent forces,” including children’s safety and the power of elites, said Alice Marwick, a disinformation expert at the University of North Carolina at Chapel Hill. “But now there is so much scaffolding from people who have researched it, it wasn’t hard for others to pick up from there.”

PizzaGate is reaching a level that nearly exceeds its 2016 fever pitch, according to an analysis by The New York Times. TikTok posts with the #PizzaGate hashtag have been viewed more than 82 million times in recent months. Google searches for PizzaGate have skyrocketed.

In the first week of June, comments, likes and shares of PizzaGate also spiked to more than 800,000 on Facebook and nearly 600,000 on Instagram, according to data from CrowdTangle, a Facebook-owned tool for analyzing social interactions. That compares with 512,000 interactions on Facebook and 93,000 on Instagram during the first week of December 2016. From the start of 2017 through January this year, the average number of weekly PizzaGate mentions, likes and shares on Facebook and Instagram was under 20,000, according to The Times’s analysis.

The conspiracy has regained momentum even as its original targets — Mrs. Clinton, her top aides and a Washington pizzeria, Comet Ping Pong — are still dealing with the fallout.

Hateful comments have recently surged on the Facebook page and Yelp and Google review pages for Comet Ping Pong, where the child trafficking supposedly happened. The pizzeria’s owner, James Alefantis, said he had received fresh death threats that caused the Federal Bureau of Investigation to open a new investigation two months ago. The F.B.I. said Friday that it could not confirm the existence of an investigation.

“There are no real options for someone like me. I don’t have the names or numbers for people to call at Google or TikTok,” Mr. Alefantis said. “But I don’t want to be that person who lives their life in fear.”

Representatives for Mr. Bieber didn’t respond to requests for comment.

PizzaGate was born in 2016 in online forums like 4chan and Reddit, where right-wing users and supporters of Donald J. Trump pored over hacked emails from John D. Podesta, Mrs. Clinton’s senior campaign adviser, looking for evidence of wrongdoing. Some emails referring to Mr. Podesta’s dinner plans mentioned pizza. A 4chan participant then connected the phrase “cheese pizza” to pedophiles, who on chat boards use the initials “c.p.” to denote child pornography.

Mr. Alefantis, who is friends with Mr. Podesta’s brother, Tony, was mentioned in several of the emails. That led internet users to connect his pizza parlor to their conspiracy.

The theory soon appeared in bogus publications like The Vigilant Citizen and The New Nationalist on Facebook and Instagram. On Twitter and YouTube, other users amplified the content.

Fact checkers debunked the idea. But weeks after the November 2016 election, Edgar M. Welch, 32, a North Carolina resident, drove six hours to Comet Ping Pong to free what he believed were enslaved children. He shot several rounds from a military-style assault rifle into a locked closet door of the pizzeria and eventually surrendered to the police. In 2017, he was sentenced to four years in prison.

Soon after, YouTube, Twitter and Facebook suspended the accounts of users who had pushed PizzaGate and took down hundreds of related posts.

To keep PizzaGate tamped down, the social media companies took other steps. Facebook made it impossible to search for hashtags such as #pizzagateisreal. On YouTube, searching for #pizzagate brought up a label that explained the term was part of a false conspiracy. Twitter also stopped #pizzagate from surfacing in its trending topics in the United States.

But starting in April, a confluence of factors renewed interest.

A documentary promoting PizzaGate, “Out of Shadows,” made by a former Hollywood stuntman, was released on YouTube that month and passed around the QAnon community. In May, the idea that Mr. Bieber was connected to the conspiracy surfaced. Teenagers on TikTok began promoting both, as reported earlier by The Daily Beast.

A week ago, Rachel McNear, 20, watched “Out of Shadows,” which has garnered 15 million views on YouTube. She then turned to Twitter, where she came across Mr. Bieber’s supposed association with PizzaGate. After reading more on Instagram, YouTube and Facebook, she created a one-minute description of her research on the topic and posted it to TikTok on Monday.

“The mainstream media uses words like conspiracy theory and how it is debunked but I’m seeing the research,” Ms. McNear, of Timonium, Md., said in an interview.

Her video was taken down on Wednesday when TikTok removed the #PizzaGate hashtag and all content searchable with the term. A TikTok spokeswoman said such content violated its guidelines.

That same day, Facebook also expunged PizzaGate-related comments under Comet Ping Pong’s page after a call from The Times.

YouTube said it had long demoted PizzaGate-related videos and removes them from its recommendation engine, including “Out of Shadows.” Twitter said it constantly eliminates PizzaGate posts and had updated its child sexual-exploitation policy to prevent harm from the conspiracy. Facebook said it had created new policies, teams and tools to prevent falsehoods like PizzaGate from spreading.

Teenagers and young adults, many of whom are just forming political beliefs, are particularly susceptible to PizzaGate, said Travis View, a researcher and host of the “QAnon Anonymous” podcast, which examines conspiracy theories. They are drawn to celebrity photos on tabloid sites and Hollywood blogs to uncover PizzaGate’s supposed secret symbols and clues, he said. Even a triangle — which can signify a slice of pizza — can be taken as proof that a celebrity is part of a secret elite cabal.

“It all becomes a game, and people are drawn in because it feels participatory,” Mr. View said.

For Tony Podesta, John Podesta’s brother, PizzaGate’s revival has opened up old wounds. He had dealt with trolling from conspiracy believers in 2016. Recently, he got a voice mail message from an anonymous caller saying, “Your pizza is ready.”

“It just doesn’t go away,” Mr. Podesta said. “They are always three steps ahead of the sheriff.”

Cecilia Kang reported from Washington, and Sheera Frenkel from Oakland, Calif.