"Someone can walk up in the middle of the marathon disguised as a runner or a spectator, and we have no way of knowing, " Mr Marciani says.

BBC: Boston Marathon attack: How secure are marathons?

She had taken part in the London Marathon in 1998 as a fun runner, but in April this year she knocked an hour off her previous time, finishing three minutes inside the Olympic qualifying time as the top British woman runner.

BBC: NEWS | UK | Wales | North West Wales | Island backs runner's dream