According to CNN, British investigators have descended upon a small English town to find out how two seemingly random individuals were exposed to the same Soviet-era nerve agent that nearly killed a former Russian double agent and his daughter earlier this year.

Authorities confirmed Wednesday night that a couple, identified in British media as Charlie Rowley and Dawn Sturgess, were being treated for exposure to Novichok, a highly toxic nerve agent first developed in secret by the Soviet Union.

CNN said the pair remain in critical condition after being rushed on Saturday to Salisbury District Hospital, the same hospital that treated former double agent Sergei Skripal and his daughter Yulia who were poisoned in March.

Police say they don't believe that Rowley and Sturgess were deliberately targeted, unlike the Skripals who British authorities suspects were the victims of a Russian plot.

Rowley and Sturgess live in Amesbury, a town of fewer than 10,000 people about 20 minutes drive from Salisbury where the Skripals were poisoned. Authorities did not name the two but described them as British nationals working in the area.

Paramedics were called to an address on Muggleton Road, Amesbury, at 10:15 a.m. Saturday, after a woman, later identified as Sturgess, collapsed, according to Neil Basu, the assistant commissioner for counter-terrorism at the London Metropolitan Police. They were called back later the same day when the man, now known to be Rowley, had also fallen ill.

The couple made a round trip to Salisbury from nearby Amesbury on Friday, according to family and friends who spoke to British media.

Authorities said while they were in Salisbury they didn't visit any of the sites that were decontaminated in connection to the Skripal case.

Basu said authorities received test results Wednesday evening that confirmed the couple had been exposed to Novichok.

The priority, according to Basu, is to establish how Rowley and Sturgess came into contact with Novichok.

Novichok, which works by causing a slowing of the heart and restriction of the airways, is one of the world's rarest nerve agents. Very few people outside of Russia have experience with it.

It was first developed in secret by the Soviet Union during the Cold War in the 1980s, as a means of countering US chemical weapons defences but was revealed to the world by former Soviet scientist and whistle-blower Vil Mirzayanov.

Mirzayanov told CNN Novichok is up to 10 times as potent as VX, the weapon used to kill Kim Jong Un's half-brother, Kim Jong Nam, in 2017.