Last week the London Review of Books published an article by the respected Pulitzer prize-winning journalist Seymour Hersh, The Red Line and the Rat Line, in which he details the alleged involvement of the Turkish government with the Syrian opposition group Jabhat al-Nusra in last August's sarin attack in Damascus. Between 1,000 and 1,400 people are estimated to have died.

The US, Britain and other western governments have pinned the blame on the Syrian government; Russia has accused the rebels. Hersh describes this as part of a "false flag" operation designed to draw the US into a conflict with Syria.

In his 6,000-word article Hersh relies heavily on single, unnamed sources for each of his claims, and constructs a narrative in which the Turkish government was responsible for the largest chemical attack since the one carried out by Saddam Hussein on Halabja in 1988. But Hersh's story is full of holes, and it brings the reliability of his sources and conclusions into question.

Hersh makes no mention of the munitions used on 21 August, something that is key to understanding the attacks. In an interview for Democracy Now! he states that the weapons were both homemade and not in Syria's arsenal. Both these claims are wrong.

Two types of munitions were used on 21 August and are linked to the dispersal of sarin gas. Both were recorded in a report by the UN and the Organisation for the Prevention of Chemical Weapons (OPCW) and tested positive for signs of sarin. One was a Soviet-era M14 140mm artillery rocket, certainly not a "homemade" munition, and the second was a munition that was widely unknown.

Studying videos and photographs posted by opposition and pro-government sources has allowed researchers to piece together a great deal of information about these rockets. We know they are referred to as "Volcano" rockets and come in at least three sizes: 107mm, 122mm (as used on 21 August), and a much larger type based on a 220mm rocket motor. It is also clear that there are both explosive and chemical versions, as well as evidence of their use by the Syrian military from late 2012.

Videos and photographs from 21 August show at least eight examples of the chemical type of Volcano rockets. There are three videos from an alleged chemical attack in Adra, Damascus, on 5 August showing the same type of Volcano rockets; one video from Adra in June showing a chemical Volcano rocket; and videos and photographs from December 2012 to January 2013 showing the remains of chemical Volcano rockets. In all examples the rockets have been fired at the opposition and appear to share an identical design, down to the smallest details.

Photographs and footage going back to November 2012 filmed by opposition activists shows Volcano rockets being launched. Eight videos from Mezzeh airbase, near Daraya, from December 2012 show Volcano rocket launches; and from October 2013 the rockets and their launchers began to show up from videos posted by the Syrian government's National Defence Force. Eventually, even pro-Hezbollah sources started posting images of Volcano rockets in action. It is quite clear that no matter how "homemade" Hersh believes the rockets to be, they have been used by the Syrian military for more than a year.

When questioned about this in a recent interview with the Turkish website Diken Hersh dismissed the Volcano rockets, seemingly because he believes a range of "a mile" somehow means they should be discounted as important to his narrative. Hersh refers to the work of Ted Postol and Richard Lloyd who believe the range of the rockets is about 2km. But this range issue isn't the problem Hersh appears to think.

Video footage from both sides of the conflict has allowed researchers to accurately find the positions of government controlled areas on 21 August. The Russian-language news site ANNA News posted two dozen videos showing "Operation al-Qaboun", a Syrian government military operation running from June to August 2013. Embedded with Syrian forces, they were able to film the progress of the operation to clear positions between Jobar and Qaboun, a strip of land about 2km away from the 21 August impact sites.

Videos from opposition groups show the other side of the fighting, including attacks on checkpoints and government movements. Based on this information it appears that the front lines were about 2km away from the furthest impact sites of rockets used on 21 August. Only two rockets landed at the 2km maximum range described by Lloyd and Postol, with the reported impact sites of the remaining rockets being between 1.5km-1.8km away. It has been possible to confirm the precise impact locations of some of the rockets by a combination of GPS information, satellite map imagery, photographs and videos, and around a dozen impact sites were reported by local groups in eastern Ghouta. Hersh's belief that the 2km rocket range is enough to dismiss them from his narrative is clearly misguided.

Despite Hersh's claims we can clearly see that the rockets were used by the Syrian government, and within range of government controlled territory.

Seven issues to address

But what about his claim of Turkey helping Jabhat al-Nusra, which is linked to al-Qaida, to produce sarin in Aleppo? Seven issues need to be addressed: difficulty of manufacture, quantity, choice of weapon, economics, logistics, concealment, and the specificity of the product.

Firstly, sarin is difficult to make. Anyone who claims otherwise is oblivious to both history and chemical engineering. Germany, the US and the former Soviet Union took years to perfect the process. Its production requires a number of complex steps and the ability to handle highly dangerous chemicals at very closely controlled high temperatures and pressures. There is no evidence anyone has come up with any sort of streamlined method to manipulate the molecules to make sarin.

Second, quantity. Perfecting the process isn't enough – there is a difference between making a spoonful and enough for the August attacks, which would have needed about half a ton. This assumes a scale only reached by big state production programmes. To put it in perspective, the one verified example of non-state production of sarin was the Aum Shinrikyo cult in Japan. Their many millions of dollars, very large purpose-built manufacturing facility and highly qualified staff got them the ability to make single batches of perhaps 8 litres of short shelf life Sarin. The alleged Aleppo plant wouldn't need to be the size of the Rocky Mountain Arsenal in the US, but it would have needed to be closer to that than the size of a house.

Third is the choice of weapon. Of the panoply of chemical warfare agents available to modern science and engineering, sarin is one of the hardest to make. So why was this one chosen? Even its nerve agent kin, Tabun and VX, are arguably easier to produce; mustard or lewisite are easier and use less technology. Numerous toxic industrial chemicals which might "fly under the radar" of non-proliferation regimes could be used as weapons. So why pick the hardest?

Fourth, economics. To make this operation work it is going to take a lot of highly trained people, raw materials, and specialised equipment, as well as a facility. It would cost many tens of millions of dollars. When the rebel factions are so stretched for resources, does it make any sense to spend, say, $50m on a weapon of limited utility? Lavish expenditure must raise a paper trail somewhere; there would be order books and receipts. Let's see them.

Fifth is logistics. You don't turn precursor material magically into sarin: you need about 9kg to end up with 1kg of sarin. This stuff has to come from somewhere, but where? Hersh omits these details, as do most of the alternative narratives. It is simply assumed that things like phosphorus trichloride and thionyl chloride just get summoned up in vast quantities without someone noticing. Most commentators on this issue have also forgotten about something called conservation of mass. If you use 9kg of material to synthesize 1kg of sarin you end up with 8kg of waste, rather a lot of which is very dangerous, smelly and corrosive. This waste stream has to go somewhere, and someone will notice. There are also the logistics of getting a lot of sarin into rockets and getting those rockets from Aleppo to Damascus.

Sixth, concealment. How do you hide all of this? The building, the supply chain, the people, the money, and the very smelly waste stream. Where are they? They need to be concealed not just from the Syrian regime but from other rebel factions, western intelligence agencies, the Russians, and perhaps even your own people who might desert, get captured or say silly things on YouTube videos. There is deathly silence from Aleppo and we only find out about it from Hersh?

Lastly is the specificity of the product. There are important physical clues found in the traces of sarin at the impact sites of the 21 August rocket attack. One of these is the presence of hexamine, a chemical with no history of use in nerve agent production. But hexamine can be used as an acid scavenger, and thus its presence could be explained due to its use as an additive to the sarin. This idea has been reinforced by both the admission of the Syrian regime that they used hexamine as part of their formula, and by Syria's declaration to the OPCW of an inventory of 80 tonnes of hexamine, specifically as part of their chemical weapons program. Surely, as an uncontrolled substance, they could have omitted it from their declarations. But they didn't. Hexamine in field samples plus hexamine in Syrian inventories, plus an admission that hexamine was in their recipe, seems a compelling case for tying the Sarin in the field to the Syrian regime. How would an Aleppo-based rebel factory somehow come up with the same exact idea?

Taken cumulatively, all these points add up to a very high degree of improbability. Isn't it more probable that the Sarin came from the people who confessed to having a Sarin factory, fired from areas controlled by the government 2km away from the impact sites, in munitions the government had been using since 2012?