An acupuncturist who treated Claire Squires has told an inquest into the London Marathon runner's death that she detected an irregular heartbeat in her client prior to the race.
Stephanie Cernoe said she advised her to see a GP.
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.