Four minutes into a video that was posted on Instagram last month, Justin Bieber leaned in to 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 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 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 Bieber’s action. The videos were translated into Spanish, Portuguese and other languages, amassing millions of views. Fans then left thousands of comments on 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, before 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 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 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.