Porton Down: What is the experimental government facility in Wiltshire at the centre of recent poisonings?

The major incident in Amesbury saw two people poisoned by the same nerve agent that almost killed the Skripals, government scientists have confirmed. The attack turns attention once more to Porton Down, the mysterious laboratory that has unintentionally become central to the response to the attacks.

The secretive government facility at Porton Down has been used for experiments involving deadly and often undisclosed weapons, and in the wake of the Salisbury attack has become indelibly associated with the nerve agent used in the attack. The Met – whose counter-terror police are now leading the investigation – confirmed that samples had been tested at the facility and that they showed "show the two people have been exposed to the nerve agent Novichok".

Porton Down is often talked about in the singular, but is actually a site located near Porton village that is host to a whole group of different organisations. The two key ones are the Defence Science and Technology Laboratory, which is run by the Ministry of Defence and usually referred to as Dstl, as well as Public Health England – both bodies have been involved in the response to the recent poisonings, though it is the former laboratory whose activity is most mysterious.

In the new case, authorities had initially thought that the two patients had become ill because of contaminated drugs. But, this week, samples were sent to Porton Down for identification – and the findings threaten to cast yet more scrutiny on the nerve agent as well as the laboratory that has been central to identifying and understanding it.

"From initial assessment it was thought that the two patients had fallen ill after using drugs from a potentially contaminated batch," said the Met's assistant commissioner of specialist operations, Neil Basu.

"However, on Monday, 2 July, due to concerns over the symptoms the man and woman were displaying, samples from both patients were sent to Porton Down laboratory for analysis.

"Following the detailed analysis of these samples, we can confirm that the man and woman have been exposed to the nerve agent Novichok, which has been identified as the same nerve agent that contaminated both Yulia and Sergei Skripal."

Porton Down would be notable in any use of nerve agent on British soil, given its importance in testing and understanding such deadly weapons. But it is also by coincidence literally close to the two attacks: Salisbury, Amesbury and the facility are all just a few miles apart, in a triangle that has now become central to the attacks and the fallout from them.

That proximity has even led to conspiracy theories suggesting that there is more to the story than authorities are letting on. But there is absolutely no proof that the location is anything more than a coincidence, or that the closeness of the site to the poisonings is at all relevant.

Porton Down has been the subject of conspiracy theories as long as the British public have known about it. Some real incidents have helped to sew doubt about – but it has also been the subject of much wilder false rumours, and the government now hosts a website that includes denials that the facility is a cannabis farm and which states "no aliens, either alive or dead have ever been taken to Porton Down".

But such conspiracy theories have flourished in the wake of the first attack in Salisbury, in the midst of confusion about what happened during the attack and how it was identified. Because the government has always been very secretive about the site as well as the other intelligence it claims shows Russian involvement in the attack, it has been hard for the government to deny the reports, but there is absolutely no evidence that the facility has been involved in anything other than testing samples of the nerve agent to identify that it was used.

That same secrecy has also meant that the usually private staff at the facility have been dragged into political disputes. In the wake of the Skripal poisoning, for instance, the Foreign Office said that government scientists had traced the specific sample of Novichok back to Russia – but representatives of the facilities were very clear that they had only been able to identify the nerve agent, not where it had been created or who it had been used by, leading to confusion and dispute between politicians.

That same argument is likely to begin again, as authorities attempt to find the source of the nerve agent that struck down the couple from Amesbury. It is also like to become more complex as Porton Down and other experts also try to ascertain whether and how the two sets of poisonings are related.

Mr Basu made clear that the police is "not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to". He said the possibility that these two investigations might be linked is clearly a line of enquiry for us", but that it would be important "that the investigation is led by the evidence available and the facts alone and we don’t make any assumptions".

As well as the laboratory at Porton Down, the new developments will turn fresh attention to other organisations, including Public Health England. That body had looked to assure the public that there was no threat from the substance after the Skripal attack was dealt with – and continues to do so, on the basis of the "number of casualties affected".