A retired Russian spy, convicted in his homeland of selling secrets to British intelligence, is critically ill alongside his daughter in hospital in England after being exposed to an unknown substance.
Sergei Skripal, 66, and Yuila, 33, were found unconscious on a bench in a shopping centre at Salisbury in England, where Skripal was granted refuge after a 'spy swap' in 2010.
Tests are underway to see what put the pair into intensive care, where they are both fighting for their lives.
Several members of the emergency services involved in the investigation were also sent for tests and one remains in hospital, Wiltshire police said in a statement issued around noon on Tuesday.
Eyewitness Jamie Paine raised the alarm in the southern English city.
"It was like her body was dead," he said, of the woman, who police says was known to Skripal.
"Her legs were really stiff... you know when animals die, they have rigor mortis. Both her legs came together when people pulled (her), and when she was on the floor her eyes were just completely white. They were wide open but just white and frothing at the mouth. Then the man went stiff: his arms stopped moving, but he's still looking dead straight."
Authorities say there is no known risk to the public. But police closed the local Zizzi pizza restaurant as a precaution. A local pub was also closed after investigators in yellow chemical suits cleaned down the street outside.
A worker at the shopping centre, Richard Eyre, told Euronews he had seen the aftermath of the incident and shared a photograph of the scene on Monday morning..
"I saw and heard the air ambulance helicopter, Sunday afternoon or evening, and the fire service in protective suits decontaminating the area."
The Russian embassy in the UK said it was "seriously concerned" by media reporting linking Moscow with the killing that "takes [the] anti-Russian campaign to a new level".
President Vladimir Putin's spokesman said Russia was ready to co-operate in the investigation. However, "we have no information about ... probable causes, what this man has been doing, and what this is about," Dmitry Peskov told reporters.
Speaking at The Security and Counter Terror Exposition in London, a former head of Scotland Yard's counter-terrorism command, Richard Walton, said: "The investigation must take its course but if this is state-sponsored terrorism and it looks entirely possible, then it will have grave consequences for UK Russia bi-lateral relations. Relations that are already at breaking point. The UK cannot and will not tolerate state-sponsored terrorism of any kind."
Marina, the widow of Alexander Litvinenko, who was poisoned 12 years ago in London, told Britain's Telegraph newspaper that she fears similarities to that murder.
Litvinenko, an ex-KGB agent was killed with radioactive polonium-210 - an act a British inquiry said was probably approved by President Vladimir Putin.
Ben Emmerson, a lawyer for Marina Litvinenko, the widow of Alexander, said he wanted the police to fully investigate the incident, adding: "For now it looks very much like an FSB operation designed to punish a perceived traitor. If it looks like a duck and quacks like a duck then it probably is a duck.”
The Kremlin has always denied any involvement in Litvinenko's death.