Mi5

From 911Encyclopedia:

As Independent UK reported on May 18th, 2002, the "shoebomber" Richard Reid (->) had been on a watch list by the MI5: Bush was given hijack warning by British intelligence Americans were told of attack threat, say MI5 and MI6, as Marines begin new operation against al-Qa'ida "...President Bush's CIA briefing last August about possible attacks on US targets by al-Qa'ida ­ the focus of the current "what did he know and when did he know it" furore ­ was based on British intelligence reports, officials on both sides of the Atlantic said. Intelligence sources in London confirmed yesterday that both MI5 and MI6 had sent reports to the United States in the run-up to 11 September, suggesting that America was under threat. Source: http://news.independent.co.uk/world/americas/story.jsp?story=296257