A string of deaths on UK soil are to be reinvestigated by the police and MI5 after claims of Russian involvement, Home Secretary Amber Rudd says.
BuzzFeed News claimed to have evidence of 14 deaths that were actually murders by the Russian state or mafia allies.
In each of the cases, police investigations and inquests did not find evidence of any crime.
Russia has denied being involved in the poisoning of former spy Sergei Skripal and his daughter last week.
The deaths that Buzzfeed identified include those of Boris Berezovsky, an outspoken critic of the Kremlin and a friend of Alexander Litvinenko - whose death from radiation poisoning caused a long-running diplomatic spat between the UK and Russia - and whistle-blower Alexander Perepilichnyy.
Mr Perepilichnyy died while jogging in 2012. Police said they found no evidence he was poisoned. An inquest into his death resumes in April.
Despite Ms Rudd's statement, Surrey Police said it would not be reopening the investigation into Mr Perepilichnyy's death.
Mr Berezovsky was found hanged in his bathroom in 2013. Police said a post-mortem showed no sign of a violent struggle.
BuzzFeed News reported that US intelligence sources believe these deaths were two of 14 assassinations linked to Russia.
In a letter to MPs, Ms Rudd said the government "takes seriously any suggestion that a foreign state has engaged in murder on UK soil".
Yvette Cooper, Labour MP and chairwoman of the home affairs select committee, had written to Ms Rudd last week asking for the investigations to be reopened.
She welcomed the decision to do so, saying: "Given the gravity of these issues, it is... right that the authorities should reassure us that they have looked at any further allegations or relevant evidence put forward in any other cases."
She added: "The government must satisfy itself that the correct finding was reached in each case and the public need to know that relevant questions about wider Russia links have been investigated and answered."