In late December, a 26-year-old construction worker in Sarasota County, Florida, used his phone to send a flurry of ominous online posts.

Alexander Lightner, tapping away on his Samsung Galaxy, announced his intention to commit mass murder, according to federal court records. He used the coded language of a new breed of neo-Nazis who call themselves Accelerationists. Lightner wrote that he planned to become a “saint” — the term followers use for someone who advances their racist cause through lethal acts of terror — and to set a new “Highscore,” or death toll.

Lightner launched what federal prosecutors allege were threats on Telegram, the sprawling, no-holds-barred platform that has become a hive for the movement. Accelerationists aim to speed the collapse of modern civilization and create a white ethno-state from the ashes of today’s democracies. Deep in the chatter of the platform’s roughly 900 million users, these extremists have created a constellation of Telegram channels where they encourage followers like Lightner to assassinate political leaders, sabotage power stations and railways, and commit mass murder.

A week after firing off his alleged threats on Telegram, Lightner woke up from a nap at his home to his father’s shouts: “Whoa, whoa, whoa. What’s this? Are these people here for us?”

Lightner threw an illegal, homemade silencer into a laundry basket, according to a summary of his interview with federal agents. Then he stepped into the sunlight. In his front yard, agents in camouflage and body armor pointed rifles at him. An armored vehicle faced his family home, its massive battering ram aimed at the front door.

An FBI agent asked Lightner if he knew why federal agents were at his door.

Lightner answered simply: “Telegram,” according to court records.

Late last month, Telegram burst into the news with another arrest related to alleged criminal activity on the giant messaging and social media platform. This time, the man in police custody was the company’s founder, Pavel Durov. French authorities detained the Russian-born billionaire after his plane touched down at an airport a few miles north of Paris.

French prosecutors issued preliminary charges against Durov last Wednesday related to alleged criminal activity on his platform. The allegations include organized fraud, drug trafficking and possession of pornographic images of minors, as well as refusal to cooperate with authorities, according to a press release by the Paris public prosecutor.

David-Olivier Kaminski, a lawyer for Durov, could not be reached for comment. French news reports quoted him saying that it was “totally absurd to think that the person in charge of a social network could be implicated in criminal acts that don’t concern him, directly or indirectly.”

The platform Durov created has long been both applauded and derided for its extreme commitment to free speech and for rebuffing inquiries from both U.S. and foreign law enforcement agencies, which have sought to gather information about alleged criminal activity on the platform.

“They are exceedingly unhelpful,” said Rebecca Weiner, the New York Police Department’s deputy commissioner of intelligence and counterterrorism. Weiner, who oversees one of the world’s largest metropolitan counterterrorism units, said the platform was notable for “being a center of gravity for a wide range of extremist content” and for its “unwillingness to work with law enforcement.”

Telegram’s ease of use, its huge public channels and the ability to encrypt private conversations have helped fuel its global appeal. Ukrainian President Volodymyr Zelensky used the app to rally his compatriots to repel the Russian invasion. Activists in Hong Kong turned to Telegram to organize demonstrations against a repressive law. In Belarus, pro-democracy forces used the platform to fight back against election fraud.

But the platform has also served as the online home of the Russian mercenary company Wagner Group, which has posted gruesome videos of extrajudicial killings. In April, the British government targeted the Terrorgram Collective, a subset of Telegram users who promote racially and ethnically motivated terrorism to people like Lightner, making it a crime to support or belong to the group. And more recently, the service played a key role in fomenting the anti-immigrant riots that swept across the United Kingdom.

ProPublica and FRONTLINE have been investigating Telegram’s role in a string of recent alleged far-right acts of sabotage and murder, and how the company’s inaction allowed extremists to plan and even advertise their crimes. Researchers have long warned that Telegram routinely allows extremists to share propaganda aimed at inciting violence, noting that the Islamic State group and al-Qaida were able to use the service for years with little interference.

“Telegram plays a key role in the perpetuation of militant accelerationism,” said Michael Loadenthal, a research professor at the University of Cincinnati and director of the Prosecution Project, which tracks felony cases involving political violence in the U.S. The company, he said, “has shown that deplatforming violent and hateful content is not its priority.”

Before Durov’s arrest, a Telegram spokesperson responded to questions from ProPublica and FRONTLINE in messages on the platform. The spokesperson said that the company bars users from calling for acts of violence, adding that moderators remove millions of pieces of harmful content from the platform every day. “As Telegram grows, it will continue to solve potential moderation problems with efficiency, innovation and respect for privacy and free speech,” the spokesperson, who used the name Remi Vaughn, said in the messages.

Yet ProPublica and FRONTLINE found that Telegram today is the main nexus of far-right Accelerationist crime. Law enforcement agencies on both sides of the Atlantic have interrupted a series of criminal schemes, including:

• In July, a Georgian man accused of leading an Accelerationist terror group was arrested in Europe for allegedly soliciting people to carry out murders and bombings in the U.S. Michail Chkhikvishvili allegedly used Telegram to communicate and distribute his group’s propaganda and is facing charges in New York. He is being held in Moldova pending extradition, according to Wired. ProPublica and FRONTLINE could not locate counsel for him.

• The same month, federal prosecutors charged an Accelerationist named Andrew Takhistov with plotting to destroy an energy facility in New Jersey. They allege he used Telegram to incite racial violence and share a how-to guide for white supremacist terrorism that included instructions on the use of Mylar balloons and Molotov cocktails to damage power substations. An attorney for Takhistov did not respond to a request for comment.

• In June, Manhattan prosecutors announced charges against Hayden Espinosa, accusing the Texas man of selling illegal guns and firearm components through a Telegram channel aimed at white supremacists and Accelerationists. Espinosa allegedly used a contraband phone to sell weapons and gun parts while incarcerated in federal prison. He has pleaded not guilty.

• A judge in England recently sentenced a British man to eight years in prison for plotting to carry out a suicide bombing at a synagogue. According to the Crown Prosecution Service, 19-year-old Mason Reynolds was “the administrator of a Telegram channel which shared far right extremist, antisemitic and racist views, as well as manuals on bomb building and how to 3D print firearms.”

• Brandon Russell, a former leader of the Atomwaffen Division, a now-defunct neo-Nazi group tied to five murders, was charged last year with planning an attack aimed at disabling the power system in Baltimore. Russell and a co-defendant, Sarah Beth Clendaniel, used Telegram to organize the sabotage scheme, according to prosecutors. Clendaniel has pleaded guilty; Russell faces trial later this year. Attorneys for the duo declined to comment.

And then there is Lightner. U.S. prosecutors say in court filings that Lightner went to Telegram to discuss his plans to use a .308-caliber rifle to kill as many people as possible. He remains in jail awaiting trial on federal charges of making threats online and possessing an illegal silencer. He has pleaded not guilty. His attorney declined to comment.

Before Lightner’s arrest, he told an agent from the Bureau of Alcohol, Tobacco, Firearms and Explosives that he was “blackout drunk” at the time of the posts, distraught over a bad breakup. “I was broken and really upset. And I went drinking, and then I did some stupid thing online,” he said, according to a recording of the conversation. He told other agents that he was not planning an act of violence but just wanted someone to notice him and care.

Lightner told federal agents that he started using Telegram in 2015, about two years after the platform launched. The online service grew steadily over the next few years, with the majority of users coming from outside the U.S. Then in 2021, Telegram’s growth exploded after its rival WhatsApp announced a new privacy policy. Some users feared WhatsApp was poised to begin sharing their confidential messages with parent company Facebook, now called Meta. In a Telegram post, Durov boasted that his platform was experiencing “the largest digital migration in human history,” claiming that 25 million new users joined Telegram in 72 hours.

That same month, in the U.S., Telegram got a bump in users when major social media platforms including Facebook and Twitter ousted former President Donald Trump and many of his most ardent supporters in the aftermath of the Jan. 6 insurrection. Today, Telegram is heavily favored by right-wing extremists, including QAnon followers, Proud Boys, militia members, and white supremacist groups like Patriot Front and the Active Clubs.

Axel Neff, who helped start Telegram, said the company’s core team of about 60 employees, 30 of whom are engineers, is too small to monitor the platform for criminal conduct. “Think about the size of Telegram. There are about a billion users on Telegram every month. A billion!” he said. “Telegram is a massive, massive community. … They are not staffed — and they do not have the capacity — to monitor everything that goes on there.”

Neff said it would be “professional suicide” for Telegram, which has marketed itself as a bastion of unfettered speech, to make a serious effort to moderate content. “I don’t think it is something [Durov] will ever do.”

The company’s privacy policy puts strict parameters around cooperation with law enforcement: “If Telegram receives a court order that confirms you’re a terror suspect, we may disclose your IP address and phone number to the relevant authorities. So far, this has never happened.”

Telegram ignores requests for information from government agencies that aren’t “in line with our values of freedom of speech and protecting people’s private correspondence,” Durov told Tucker Carlson in an interview with the former Fox News host earlier this year. Durov noted that Telegram refused to cooperate with the U.S. congressional committee probing the events of Jan. 6, 2021.

Telegram stores “very limited data” on its users, the Telegram spokesperson told ProPublica and FRONTLINE. “In most cases it is impossible for Telegram to access this data in order to provide it for the authorities,” the spokesperson said. “Police, governments and users are able to report content to Telegram they believe is illegal. Telegram processes these reports according to its terms of service.”

ProPublica and FRONTLINE found that much of the most disturbing content is posted in channels maintained by violent, right-wing Accelerationists, whose ideas have attracted neo-Nazis, Charles Manson admirers and anti-government revolutionaries.

The Terrorgram Collective, the group of Telegram users targeted by the British government’s crackdown, is an alliance of Accelerationists who use an ever-evolving array of Telegram channels to promote terrorism. The group has produced at least three e-books, including a manual celebrating white supremacist mass killers that court documents show was found at Lightner’s home in Florida.

David Skiffington, a former British counterterrorism specialist for London’s Metropolitan Police, said the “proliferation of extremist content” on Telegram “cannot be overstated.”

Other social media platforms such as Steam, Discord and Gab also host extremist-related content, Skiffington said. “But Telegram is by far the most widely used and accessible.”

Skiffington, who now runs the counterterrorism consulting firm DBA Insights, has been monitoring the Terrorgram Collective for years. He said the group’s influencers encourage “angry, white, lonely vulnerable individuals … to commit real-world acts of violence.”

It’s unclear how many people are part of the collective, though law enforcement has arrested individuals in Slovakia, Canada and the U.S. who are allegedly linked to the group.

In Florida, Lightner — or someone using his username, “Death.” — participated in at least 17 extremist Telegram channels, according to an analysis by Miro Dittrich, a co-founder of the Center for Monitoring, Analysis and Strategy, a German organization that studies online disinformation and extremism. Three of the channels were part of the Terrorgram network.

On the day of his arrest, Lightner was asked by a federal agent to explain his most explosive Telegram postings. At first, Lightner said he did not remember the online threats. But when a federal agent read the words back to him, Lightner said he had never seriously considered an act of violence. But he added that he knew that in making the Telegram postings, he was “playing with fire.”