They knew about him in British intelligence, but didn't think he was a threat. "US bomb suspect 'reached out' to UK militants-report," from Reuters, January 2 (thanks to Sr. Soph):
LONDON, Jan 2 (Reuters) - British security services knew three years ago that the Nigerian accused of trying to blow up a U.S.-bound plane had "multiple communications" with Islamic extremists in Britain, a newspaper reported on Saturday.
Umar Farouk Abdulmutallab, 23, charged with trying to blow up Northwest flight 253 from Amsterdam as it approached Detroit on Christmas Day, studied in London between 2005 and 2008.
U.S. President Barack Obama says an al Qaeda affiliate based in Yemen was apparently behind the attack.
The Sunday Times quoted counter-terrorism officials as saying that, during his London stay, Abdulmutallab had been "reaching out" to extremists who were under surveillance by British security service MI5.
None of the information was passed to U.S. officials, which will prompt questions about intelligence failures before the attack, the newspaper said in an article posted on its website....
British intelligence officials had defended their decision not to flag Abdulmutallab as a possible terrorism risk, saying he was one of many youths who mixed with extremists but who were not themselves thought to be involved in plotting or supporting terrorism, the Sunday Times said.
It quoted a senior British government official as saying that the intelligence agency had conducted a quick assessment of Abdulmutallab while he was living in London and concluded he was not a threat to national security....