Jamie Allinson makes some false technical claims in his critique of Seymour Hersh (Letters, 8 May). What Hersh reports is entirely plausible, and consistent with facts that emerged from our more limited but irrefutable technical studies of the circumstances surrounding the nerve agent attack in Damascus on 21 August 2013. Our findings, which have become the basis for the ‘new’ arguments being made against Hersh by people like Allinson, and supposedly knowledgeable non-government organisations like Human Rights Watch and the New York Times, raise the most serious questions about whether the White House lied about technical intelligence associated with the attack.

Allinson is correct that the improvised rockets he calls Volcanoes each contained about fifty litres of sarin, but wrong in his claim that they were fired from a regime-held area ‘to the north’. These claims are not original, but repeat those of Eliot Higgins, a blogger who, although he has been widely quoted as an expert in the American mainstream media, has changed his facts every time new technical information has challenged his conclusion that the Syrian government must have been responsible for the sarin attack. In addition, the claims that Higgins makes that are correct are all derived from our findings, which have been transmitted to him in numerous exchanges.

Before we began reporting findings from our analyses, there were published reports estimating that the sarin load carried by the rockets was about five litres. We showed, from detailed engineering analyses of rocket debris, that the rockets contained as much as fifty litres. This finding was hailed by members of the US government and non-government organisations, such as Human Rights Watch and the New York Times, as proof that the Syrian government had executed the atrocity of 21 August. In a follow-up analysis, we found that it could not possibly have been the case that the deadly rockets were fired from Syrian government-controlled areas as far as ten kilometres away, as claimed by the US government and non-government organisations. We showed that the shape of the rockets resulted in extreme aerodynamic drag, limiting their range to about 2 to 2.5 kilometres. This finding was met with great resistance in the media.

We also analysed the impact debris from the single rocket for which data was available (there is no data for multiple rocket impacts despite Allinson’s claim). We showed that those who argued that the Syrian government had fired the rockets had incorrectly determined the direction of arrival as being from the northwest. We showed that the actual direction was from the north. This new technical insight quickly prompted a new ‘discovery’. There was a checkpoint to the north, close to the area controlled by Syrian government forces, from which the deadly short-range rockets could have been launched. However, if they had been fired from this location, the impact pattern of the rockets used in the attack would have required them to have a range well in excess of five kilometres – which we have shown cannot be the case.

We do not claim to know who was actually behind the attack of 21 August in Damascus. But we can say for sure that neither do the people who claim to have clear evidence that it was the Syrian government. The mainstream American media have done a disservice to the public by allowing politically motivated individuals, governments, and non-government organisations to misrepresent facts that clearly point to serious breaches of the truth by the White House.