The Salisbury poisoning suspects are believed to have been identified in one of the largest and most complex investigations ever carried out by British counter-terror police.
A huge inquiry was launched after former spy Sergei Skripal and his daughter Yulia were found slumped on a bench in the Wiltshire city on March 4.
The probe was quickly taken over by Scotland Yard’s specialist counter-terror unit, while the British security services got to work behind the scenes.
By last month 250 detectives from the country’s counter-terror network had been involved in the “painstaking” investigation, according to the Metropolitan Police.
Investigators have trawled through thousands of hours of CCTV footage seeking to identify all pedestrians, cyclists and vehicles that passed through areas significant to the probe.
With Russia publicly blamed for the attack, manifests of flights into UK airports along with CCTV footage would have been checked in a bid to identify the would-be assassin or assassins.
Around 100 counter-terror detectives were still in Salisbury when Dawn Sturgess, 44, and her partner Charlie Rowley, 45, fell ill after coming into contact with Novichok.
The fresh investigation was hampered by the hot weather with work limited to 15-30 minute stints by those carrying out searches wearing protective suits.
Police were able to speak to Mr Rowley when he regained conscious following the death of Ms Sturgess and likely gained vital information about the source of the nerve agent.
It is understood investigators are working to the theory that the substance was in a discarded perfume bottle found by the couple and Ms Sturgess sprayed Novichok straight on to her skin.
The breakthrough, reported on Thursday, raises the prospect that investigators could track the suspects’ movements to and from Salisbury and in and out of the country.
It could also lead to identifying phones or other communications devices and provide an answer to how the Novichok was brought into the UK.
Tests are still ongoing to establish whether the same batch of nerve agent poisoned the Skripals and the couple in Amesbury, and the ongoing investigation will take months to complete.
Even if a strong case can be built against the suspects there is still no guarantee they will face justice.
The inquiry into the murder of the dissident Russian spy Alexander Litvinenko found two Russian men – Andrei Lugovoi and Dmitri Kovtun – had deliberately poisoned him by putting polonium-210 into his drink at a London hotel.
But Russia has refused to extradite them.
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules here