American intelligence officials said Wednesday that the men accused of trying to blow up several airliners bound for the US had no obvious links to support networks in the United States but warned the greatest threat may be lurking within the US Joseph Billy, the FBI's assistant director of counterterrorism, said there were three main levels of threat. The top tier included traditional al-Qaida cells whose newfound sanctuaries in Pakistan and elsewhere had led to renewed capabilities. The other two tiers included al-Qaida franchises and radicalized homegrown extremists inspired by al-Qaida but with no formal links. Unlike the 9/11 terror attacks where hijackers had lived and trained in the United States, the trans-Atlantic plot appeared to have prevalent links in Britain and Pakistan. "We were very hungry for information that could link them (the alleged airline plotters) to support networks in the US," Billy said at Britain's first national counterterrorism conference. "We did not see it ... Does that mean they don't exist? I'm not so sure of that."