A Russian former double agent and his daughter are still in a very serious condition five days after they were struck down by a nerve agent in a quiet English city, Britain's interior minister says.
Sergei Skripal, 66, who passed Russian secrets to Britain, and his daughter Yulia, 33, have been in intensive care since they were found slumped unconscious on a bench on Sunday afternoon in the cathedral city of Salisbury.
As counterterrorism police continued to investigate the source of the nerve agent, Home Secretary Amber Rudd visited the city on Friday, including the area around the bench - now covered by a police forensics tent - where Skripal was found.
"It is still very serious for the two people who were the subject of this outrageous attack," Rudd said.
One hundred military personnel with expertise in dealing with chemicals have been sent to Salisbury, police and the defence ministry say.
The military was asked to help because they had "the necessary capability and expertise", the Metropolitan Police said on Friday.
Police said Skripal and his daughter were deliberately targeted with the rare toxin.
They said experts had identified the substance, which will help determine the source, but have not yet disclosed to the public what it was.
The attack has been likened in Britain to the assassination of ex-KGB agent Alexander Litvinenko, a critic of Russian President Vladimir Putin, who died in London in 2006 after drinking green tea laced with radioactive polonium-210.
Britain has said it will respond appropriately if evidence shows Russia was behind the poisoning.
The Kremlin denies any involvement in the incident and says anti-Russian hysteria is being whipped up by the British media.
Russian Foreign Minister Sergei Lavrov said on Friday that Britain's warnings of retaliation if it was proven Russia was behind the poisoning were propaganda and not serious.
Twenty-one people were taken to hospital following the incident but only one other person, Detective Sergeant Nick Bailey, who was the first police officer on the scene, is still being treated.
He remained in a serious condition although he was now speaking, Rudd said.
She declined to give any details about the police investigation, saying detectives needed space to determine how and where Skripal and his daughter were poisoned.
Police have cordoned off Skripal's home in Salisbury and officers were guarding the area where he and his daughter were found, along with a pizza restaurant and a pub they had visited and the graves of Skripal's wife and son.
Skripal betrayed dozens of Russian agents to British intelligence before his arrest in Moscow in 2004.
He was sentenced to 13 years in prison in 2006, and in 2010 was given refuge in Britain after being exchanged for Russian spies.
Australian Associated Press