Scientists at the government’s Porton Down defence laboratory were carrying out tests on Wednesday night to try to establish if a couple had been exposed to a nerve agent in Wiltshire.

A park in Salisbury five minutes’ walk from the bench where the former Russian spy Sergei Skripal and his daughter Yulia collapsed after a suspected Russian attack employing a nerve agent was one of at least seven areas cordoned off in the cathedral city and the nearby town of Amesbury.

Counter-terrorism officers joined the investigation into the incident, which was initially treated as a drugs issue. In Whitehall government officials held meetings of the Cobra committee to discuss developments.

Some friends and neighbours of the couple, British nationals Dawn Sturgess, 44, from Salisbury, and Charlie Rowley, 45, of Amesbury, said they believed they had been exposed to a nerve agent. They remained critically ill at the Salisbury district hospital, where the Skripals were treated after being exposed to the nerve agent novichok.

The pair were in Queen Elizabeth Park, in Salisbury, the night before they fell ill, leading to speculation that they could have come into contact with residue of the novichok used in the Skripal attack. Police sources dismissed as speculation one theory that the Skripal nerve agent could have been prepared in the park ahead of the attack.

Facebook Twitter Pinterest Sam Hobson talks to the media outside the Amesbury Baptist Centre. Photograph: Geoff Caddick/AFP/Getty Images

At a press conference in Amesbury, near Stonehenge, deputy chief constable Paul Mills said that when the couple fell ill on Saturday it was initially believed they had been using drugs, possibly from a contaminated batch.

But a major incident involving 12 agencies was announced in the early hours on Wednesday when health officials and police became alarmed at the couple’s symptoms and were unable to pinpoint the cause.

Mills said: “Further testing is now ongoing to establish the substance which led to these patients becoming ill. At this stage it is not yet clear if a crime has been committed. We are keeping an open mind as to the circumstances.”

He explained that, based on the number of casualties affected, it was not believed there was a significant health risk to the wider public. “It is really important to stress that this will be continually assessed as further information comes to light. At this time no one else is receiving treatment as a result of this incident.” A hotline has been set up for anyone who is concerned.

Facebook Twitter Pinterest Police at a cordoned off area in Salisbury. Photograph: Henry Nicholls/Reuters

Sam Hobson, 29, a friend of the couple, said he believed they had been struck down by a nerve agent. He said Sturgess fell ill on Saturday morning, and later that day Rowley also became sick. He said both were in hospital in isolation and he was receiving regular calls from the authorities to check that he was well. “They thought it was drugs at first. They now think it’s a nerve agent,” he said.

Hobson, a mechanic, said he was in Salisbury with the couple on Friday evening in locations close to some of the places associated with the Skripal case. He said he and the couple spent time in “Lizzy Gardens” – Queen Elizabeth Gardens – close to the river on Friday. “We were having a drink and chilling in the sun,” he said.

They visited a number of shops, including a Boots chemist store to find red, white and blue dye for their hair to show support for the England football team, as well as a mobile phone shop and a store to buy alcohol. They bought food and visited a charity shop to purchase a blanket to sit on.

Hobson said he visited Rowley’s home in Muggleston Road, Amesbury, on Saturday morning. Sturgess, who lives in Salisbury, had spent the night there. “I saw lots of ambulances and Dawn got taken out on a stretcher. She needed to be helped with her breathing,” Hobson said. Rowley came out in tears. “They said she needed to have a brain scan.”

After she was taken to hospital, Hobson and Rowley went to a Boots in Amesbury. Later they attended a free hog roast organised by the local baptist church on a green.

Hobson said: “We went back to his place after the hog roast. We were going up to the hospital. Then he started sweating. His T-shirt was soaking wet. He got up and started rocking against the wall. His eyes were wide open and red, his pupils were like pinpricks. He began garbling incoherently and I could tell he was hallucinating. He was making weird noises and acting like a zombie. I phoned an ambulance. At first they thought it was drugs but … they know now it isn’t drugs.”

He said the couple had been together for a few months and neither worked. Hobson said they were both parents.

The areas known to have been sealed off in the operation were: Rowley’s home on Muggleton Road, Amesbury; the Amesbury Baptist Centre, Butterfield Drive, Amesbury; Raleigh Garden Green, Amesbury, where the hog roast took place; Queen Elizabeth Gardens, Salisbury; Sturgess’s room at John Baker House, Rolleston Street, Salisbury; a bin outside Sturgess’s home; Boots, Stonehenge Walk, Amesbury.

steven morris (@stevenmorris20) Amesbury incident - police cordon at the baptist church. pic.twitter.com/JRzIBH2Lu2

As in the early days of the Skripal incident in March this year the government was cautious. Theresa May’s spokesman said on Wednesday: “This is an incident which understandably is being treated with the utmost seriousness.”

A spokesperson for Scotland Yard’s counter-terrorism command said: “As Wiltshire police have stated they are keeping an open mind as to the circumstances surrounding the incident.”

The counter-terrorism officers who have joined the investigation will have expertise in issues to do with nerve agents and other military-grade weapons, as well as the necessary security clearances to work with intelligence agencies.

Skripal attack: police close in on time poison was used Read more

Neighbours of Rowley expressed alarm at the incident. Amy Ireland, 32, said the estate was packed with firefighters, police and paramedics on Saturday. “We were on our way back from my 10-year-old son’s football presentation. There was a huge commotion. Cordons were set up and people were being moved back,” she said. “People thought it was a gas leak at first. Now we don’t know what to think. It’s very worrying. My son plays out here on the green. First Salisbury, now this.”

Another neighbour, Jake Murphy, said he had moved to Amesbury from Salisbury for the quiet life. “And then this happens. There are all sorts of rumours floating about that it’s to do with the Russians. But I don’t know if that’s right.”

Sergei and Yulia Skripal spent weeks in Salisbury district hospital where they were treated for suspected exposure to the Russian-created nerve agent novichok. The British government has accused Moscow of being behind the attack on them.