LONDON (AP) — Britain has seen a credible terrorist attack plot about once a year since the Sept.11 attacks — a worrying pattern as security officials brace for an array of threats ahead of next month’s summer Olympics, the head of the country’s domestic spy agency says.

Although Britain’s threat level is a notch below what it has been for much of the past decade, it is still at substantial. The level means an attack is a strong possibility,

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