Remember what I said about false positives? An interesting list from Kevin Drum.
One thing I’d add is that I’m not inclined to make too much of his being rejected for a visa from the UK, nor would I want to see a policy where being rejected from one country entails automatic suspicious treatment at all other countries, because being rejected for a visa or denied entry into a country is both highly subject to the particulars of admitting countries’ laws and usually requires little in the way of evidence. A US CBP officer can deny you entry even if you have a valid visa; they have a lot of discretion. You can be rejected for one kind of American visa while retaining your eligibility for others. And it’s not clear in this case that it would have helped much. Abdulmutallab’s visa wasn’t renewed not because they thought he was a terrorist, but because the UK cracked down on those abusing the sponsorship of student visas by starting fake colleges. (He had been a genuine student at a UK university in the past.)