What do we believe? This is the crucial democratic question. Without informed choice, democracy is meaningless. This is why dictators and billionaires invest so heavily in fake news. Our only defence is constant vigilance, rigour and scepticism. But when some of the world’s most famous crusaders against propaganda appear to give credence to conspiracy theories, you wonder where to turn.

The Organisation for the Prohibition of Chemical Weapons (OPCW) last month published its investigation into the chemical weapons attack on the Syrian town of Khan Shaykhun, which killed almost 100 people on 4 April and injured around 200. After examining the competing theories and conducting wide-ranging interviews, laboratory tests and forensic analysis of videos and photos, it concluded that the atrocity was caused by a bomb filled with sarin, dropped by the government of Syria.

There is nothing surprising about this. The Syrian government has a long history of chemical weapons use, and the OPCW’s conclusions concur with a wealth of witness testimony. But a major propaganda effort has sought to discredit such testimony, and characterise the atrocity as a “false-flag attack”.

This effort began with an article published on the website Al-Masdar news, run by the Syrian government loyalist Leith Abou Fadel. It suggested that either the attack had been staged by “terrorist forces”, or chemicals stored in a missile factory had inadvertently been released when the Syrian government bombed it.

The story was then embellished on Infowars – the notorious far-right conspiracy forum. The Infowars article claimed that the attack was staged by the Syrian first responder group, the White Helmets. This is a reiteration of a repeatedly discredited conspiracy theory, casting these rescuers in the role of perpetrators. It suggested that the victims were people who had been kidnapped by al-Qaida from a nearby city, brought to Khan Shaykhun and murdered, perhaps with the help of the UK and French governments, “to lay blame on the Syrian government”. The author of this article was Mimi Al-Laham, also known as Maram Susli, PartisanGirl, Syrian Girl and Syrian Sister. She is a loyalist of the Assad government who has appeared on podcasts hosted by David Duke, the former grand wizard of the Ku Klux Klan. She has another role: as an “expert” used by a retired professor from the Massachusetts Institute of Technology called Theodore Postol. He has produced a wide range of claims casting doubt on the Syrian government’s complicity in chemical weapons attacks.

The Guardian visited Khan Shaykhun in the aftermath of the attack – the only news organisation in the world to do so

In correspondence with the chemical weapons expert Dan Kaszeta, Postol revealed that the “solid scientific source” he used to support his theory about the origin of sarin used in Syria was “Syrian Sister”. When Postol and Susli both appeared on a podcast run by the Holocaust “revisionist” Ryan Dawson, Postol explained why he had chosen to work with her: “I was watching her on Twitter. I could see from her voice … that she was a trained chemist.” First, Postol claimed that the crater from which the sarin in Khan Shaykhun had emanated was most probably caused not by a bomb dropped from the air but by an explosive device laid on the ground (a hypothesis examined and thoroughly debunked by the OPCW report). Then he claimed that there was “no evidence to support” the notion that sarin had been released from the air, and proposed there was strong evidence to suggest that the mass poisoning had been caused by a bomb that hit a rebel weapons depot.

He further claimed that a French intelligence report contradicted the story that sarin had been dropped from a plane, as it suggested that sarin had been dropped by helicopters in a different place. (In reality, he had confused the attack in April 2017 with one in April 2013). Each of these contradictory hypotheses was patiently explored and demolished at the time by bloggers and analysts.

The Guardian visited Khan Shaykhun (also known as Khan Sheikhun) in the aftermath of the attack – the only news organisation in the world to do so. It established that there had been no weapons depot near the scene of the contamination. Surrounding warehouses were abandoned. Birdseed and a volleyball net were all that existed inside. None had been attacked in recent months. The contamination came from a hole in the road from where the remains of a projectile protruded.

But eight days after the Khan Shaykhun attack John Pilger, famous for exposing propaganda and lies, was interviewed on the website Consortium News. He praised Postol as “the distinguished MIT professor”, suggested that the Syrian government could not have carried out the attack – as he claimed it had destroyed its chemical arsenal in 2014 – and maintained that jihadists in Khan Shaykhun “have been playing with nerve gases and sarin … for some years now. There’s no doubt about that.” Despite many claims to the contrary, I have found no credible evidence that Syrian jihadists have access to sarin.

On 26 April Noam Chomsky, interviewed on Democracy Now, claimed that Postol, whom Chomsky called “a highly regarded strategic analyst and intelligence analyst”, had produced a “pretty devastating critique” of a White House report that maintained the Syrian government was responsible. Although Chomsky accepted that a chemical attack had taken place and said it was plausible that the Syrian government could have carried it out, this interview helped trigger a frenzy of online commentary endorsing Postol’s hypotheses and dismissing the possibility that the Assad government could have been responsible. The atmosphere became toxic: when I challenged Postol’s claims, people accused me of being an Isis sympathiser, a paedophile being blackmailed by the government, and a Mossad agent. But the madness had only just begun.

People accused me of being an Isis sympathiser, a paedophile being blackmailed by the government, and a Mossad agent

In June the investigative journalist Seymour Hersh published an article in the German paper Die Welt, based on information from a “senior adviser to the US intelligence community” who maintained that there had been no sarin strike on Khan Shaykhun. Instead, a meeting of jihadist leaders in “a two-storey cinder-block building” had been bombed by the Syrian air force with the support of the Russians and with Washington’s full knowledge. Fertilisers and disinfectants in the basement, Hersh claimed, could have caused the mass poisoning. (Again, this possibility was examined and discredited by the OPCW).

So which building was he talking about? I asked Hersh to give me its coordinates: the most basic evidence you would expect to support a claim of this nature. The Terraserver website provides satellite imagery that makes it possible to check for any changes to the buildings in Khan Shaykhun, from one day to the next. But when I challenged him to provide them, first he sent me links to claims made by Postol, then he told me that the images are not sufficiently “precise and reliable”. As every building is clearly visible, I find this claim is hard to understand.

Scepticism of all official claims is essential, especially when they involve weapons of mass destruction, and especially when they are used as a pretext for military action – in this case Tomahawk missiles fired on the orders of Donald Trump from a US destroyer on 7 April. We know from Iraq not to take any such claims on trust. But I also believe there is a difference between scepticism and denial. While in the fog of war, there will always be some doubt, as the OPCW’s report acknowledges, there is no evidence to support the competing theories of what happened at Khan Shaykhun. Propaganda by one side does not justify propaganda by another.

In Vox earlier this month, the writer David Roberts suggested that America is facing “an epistemic crisis” caused by the conservative rejection of all forms of expertise and knowledge. Politics in the US and elsewhere is now dominated by wild conspiracy theories and paranoia – the narrative platform from which fascism arises. This, as Roberts proposes, presents an urgent threat to democracy. If the scourges of establishment propaganda promote, even unwittingly, groundless stories developed by the “alt right”, we are in deeper trouble than he suggests.

• George Monbiot is a Guardian columnist