In the unlikely setting of a wedding venue, Antrobus House, in the Wiltshire town of Amesbury, Britain’s leading counter-terrorism officer, Neil Basu, gave an fascinating insight into the novichok investigation. Here is what we learned about the attack on the Russian former spy Sergei Skripal and his daughter, Yulia, and the nerve agent poisoning that has led to the death of Dawn Sturgess and the hospitalisation of her partner, Charlie Rowley.

Is the Russian state responsible for both incidents?

Basu said he stood by the British government’s assessment that the attack on the Skripals was either down to the Russian state or that Moscow had lost control of a nerve agent produced during the cold war. Basu said the idea that the Skripal attack in March and the poisoning now of Sturgess and Rowley were not linked was “implausible”. But he said no forensic link had yet been found, and he conceded it was possible no such connection would ever be established. Basu did not spell out how scientists at the government’s Porton Down laboratory were trying to establish a link, but he explained: “The scientists would need a certain sample to compare to the one taken from the Skripals; we don’t have that forensic link.”

Have the Skripals been able to help establish how and when they were attacked?

No. “They had no idea they were being targeted,” said Basu. “They had no idea they had been contaminated.” He said detectives were sure the pair were poisoned at Sergei Skripal’s home on the outskirts of Salisbury. “We are certain the source, ground zero, was their front door,” he said. Basu also said he was certain the Skripals’ movements at the crucial time had been traced and the areas they were in contact with – such as the bench where they collapsed on 4 March – had been decontaminated.

Has the source of the Skripal poisoning been found?

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No. The working assumption is that the novichok used in the Skripal attack was in a container that Sturgess and Rowley found and opened. They had what Basu called a “major dose”. Basu confirmed that the police had not previously searched Queen Elizabeth Gardens in Salisbury, an area the couple visited before falling ill and one of the zones cordoned off. Officers are searching Sturgess’s home in Salisbury and Rowley’s in Amesbury hoping to find the novichok there. Basu said the “brutal reality” was that police had not yet caught the would-be assassin or assassins, had not found the container and could not be certain there was no nerve agent still out there in Salisbury.

Had the police been searching for the novichok before Sturgess and Rowley fell ill?

When a member of the public told Basu his officers should have been searching for the container before Sturgess and Rowley fell ill, he replied: “I take your point. I know you are really concerned about that.” He said he had no idea what the container even looked like, and conceded there may be more than one container.

Basu said he hoped Rowley would soon be well enough to tell police where he and Sturgess found the container. He said it was possible one or both of the couple found the container at the time of the Skripal attack and only opened it recently. He refused to speculate on whether the novichok that poisoned Sturgess and Rowley had been left deliberately or whether the attackers may simply have been careless.

When were Sturgess and Rowley poisoned?

The meeting heard that the symptoms emerged between three and 12 hours after exposure. Sturgess fell ill at about 10am on Saturday 30 June at Rowley’s home in Amesbury. Rowley fell ill at about 6.20pm that day. That would suggest they were exposed early in the morning on Saturday. It was made clear that the novichok was in liquid form. Basu said he was sure the couple were not deliberately targeted.

Have traces of novichok been found anywhere else?

No. There was no trace of the poison on a bus Sturgess and Rowley caught. Officers are still testing a van seized on Sunday evening. Three men who travelled with Rowley in the van on Saturday have undergone health tests and are well. Basu said Sturgess and Rowley had not drunk or injected the substance, they had touched it and it was on their hands.

What happens if the container is not found?

Basu said it would be impossible to keep searching for ever. The hunt had to be intelligence-led. It was made clear there is no detecting device that allows a city to be swept for novichok. Rather, swabs have to be taken, transported to Porton Down and analysed there.

How long does novichok remain active?

If it is outside and exposed to the elements, it gets washed away and becomes safe. Anything left over in the outside from March would not be dangerous. But Basu said: “If it was sealed in a container … it would last probably, I’ve been told by scientists, for 50 years.” The officer said he did not believe there would be much novichok in Salisbury. “If you’re going to use something like this to try to assassinate someone, you would be foolish to carry around large amounts. It would make it easier to detect you, it’s lethal to you.”

How difficult is the search?

Extraordinarily challenging. Basu said it took experts – all volunteers – 40 minutes to change into their protective suits and 40 minutes to “de-robe”. They were working in 40C heat in the two main properties that are under investigation and could only work for 15 minutes at a time, compared to up to six hours in March. This meant they could carry out only one, two or three swabbings each session. The volunteers’ blood was tested as they went in and as they left. Teams of six to eight were working together. The search could take weeks or months. Basu said the safety of officers and the public was paramount.

Who is leading the investigation?

The south-east counter-terrorism unit is leading with support from the Metropolitan police and all the other CTUs across the UK. Basu, who is national lead for counter-terrorism policing in the UK, said: “My sole job in life is to find people who have done very bad things and lock them up. I intend to do that in this case.”