The third Russian agent implicated in the Salisbury nerve agent attack aborted his planned exit from the UK, raising the prospect he remained in the country.
The Russian military intelligence officer - using the false name Sergey Fedotov - traveled to the UK on the same day as two hitmen who carried out the attempted assassination of Sergei Skripal, The Telegraph reports.
Fedotov was then booked on to the same flight back to Moscow with the assassins. But it is now understood, according to sources, that Fedotov checked himself and his baggage off the plane before departure.
It is not clear why, but it raises the prospect that Fedotov stayed in Britain, at least in the days after the deployment of military-grade nerve agent in Salisbury. His current whereabouts is unknown.
A source told said: "It is not clear why Fedotov did not board the flight. But at the last minute he checked himself and his bags off it.
"He could still have been running around Britain."
It is possible that Fedotov may have stayed in Britain after realizing Skripal had not been killed.
Fedotov, who claimed to be 45, was using a passport whose number differed by only a few digits from the fake passports used under the names of Alexander Petrov and Ruslan Boshirov.
It later emerged that Petrov and Boshirov were Alexander Mishkin and Anatoliy Chepiga, senior officers in the GRU military intelligence service who had been awarded Russia's highest honor by President Vladimir Putin for their activities in Ukraine.
Fedotov's true identity has not been uncovered, and his role in the attack on a former Russian spy Skripal remains unclear.
One theory says he brought the Novichok from Russia and handed it to his two GRU colleagues.
Also, Fedotov could have done reconnaissance to let the other two hitmen know when the Skripals had left their home on the day of the attack.
No CCTV images of Fedotov exist, suggesting he never traveled to the Wiltshire city. Also, no traces of Fedotov have been found in documentary databases or on social media. He has no property, vehicles or telephone registered to his name in Russia, according to an acclaimed Russian news outlet Fontanka.
Fedotov's passport is of the "64 series" linked to not only Chepiga and Mishkin but also to other suspected GRU agents such as Eduard Shishmakov, accused of a failed plot to assassinate the prime minister of Montenegro.
According to Fontanka, Fedotov previously traveled to Britain in March 2016 and March 2017 and again in 2018.
He flew to the Czech Republic with Mishkin in 2014, apparently to monitor Skripal, who was said to be briefing Czech secret services. It raises the possibility that Skripal, who had lived in Britain since a spy swap in 2010, was a GRU target for at least four years.