The first police on the scene of the poisoning of the former Russian spy Sergei Skripal realised the seriousness of the case after Googling his name.
Mr Skripal and his daughter Yulia were targeted with the nerve agent Novichok on 4 March 2018 in Salisbury.
Sgt Tracey Holloway said after finding his name online they realised "it could be something bigger".
A church service, ahead of the first anniversary of the attack, has taken place.
St Thomas's Church, in the city, remembered the victims and offered thanks to the community.
Mr Skripal's house and 11 other potentially infected sites were ruled safe on Friday.
The operation included taking thousands of test samples from across Salisbury and nearby Amesbury, where Dawn Sturgess, 44, was fatally poisoned in July.
Britain has accused Russia of carrying out the poisoning of the Skripals.
Russia has also been blamed for the death of Ms Sturgess and the poisoning of Mr Rowley, who are believed to have come into contact with a bottle of Novichok discarded by the Skripals' attackers.
Sgt Holloway said they knew "relatively early that they were both going to intensive care".
But police had initially thought the Skripals' collapse had been caused by a drugs overdose.
"The paramedics said they weren't sure what it was and we didn't know what they were suffering with," she said.
"They weren't dressed in the way I would expect a drug user to be, so I wasn't really sure what we had."
When the officers went back to the station they made the link to Russia - and the possibility of a much bigger case - as they found Mr Skripal's name online.
"It was actually another CID officer who had Googled his name and then said, 'Tracey, do you want to come and have a look at this?'
"It was at that point that we got the link to the Russians side of things. And at that point we thought this could be something bigger than what we believed could be a drugs overdose."
Sergei and Yulia Skripal survived the attack following treatment at Salisbury District Hospital.
Det Sgt Nick Bailey, who searched their house following the poisoning, returned to work last year and is preparing to run a marathon to raise money for the hospital.
Appeal to Putin
Meanwhile Ms Sturgess's son has urged Vladimir Putin to hand over the men suspected of being responsible for his mother's death.
Ewan Hope has appealed directly to the Russian president "as a human being" to allow British investigators to speak to the suspects.
His mother's partner, Charlie Rowley, said he wanted "someone to pay for what they've done".
In an open letter to Mr Putin published by the Sunday Mirror, Mr Hope said: "The British police believe at least two Russian citizens were responsible for her death but it appears they are being protected by your state.
"I am appealing to you as a human being to allow our officers to question these men about my mother's murder. The least she deserves is justice."
You can watch more about how Salisbury is recovering a year after the attack on a special edition of BBC Points West on BBC One at 6.30pm on Monday and for 24 hours after on the BBC iPlayer.