Updated

American intelligence officials said Wednesday that the men accused of trying to blow up several airliners bound for the U.S. had no obvious links to support networks in the United States but warned the greatest threat may be lurking within the U.S.

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 Sept. 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 U.S.," 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."

The conference drew hundreds of British and international law enforcement officials, intelligence agents and lawyers. Two former British militants — one whom spent more than four years in an Egyptian jail for his membership to Hizb ut-Tahrir — also spoke and warned law enforcement officials that militants will change tactics once they think they've been profiled.

A host of challenges confront counterterrorism agents today, including technology advances that made communication undetectable, Internet recruiting sites, gaps in international law that limit international cooperation and the mutation of al-Qaida.

"I do not have an al-Qaida cell that I could put on the board for you," he said. "We have not seen that. We have seen individuals with some links — some indirect ties, some more direct — but to have (a) cell that is plotting and moving forward has yet to be found."

The men accused of trying to blow up at least 10 U.S.-bound airliners in 2006 are to appear in a London court in April — just one of many terrorism cases with international implications.

Billy called the airline plot "worrying" and said that U.S. officials would have been caught unaware without British investigators who discovered the plan.

Britain and the United States — strong allies in the Iraq war — face significant terror threats that show no sign of diminishment, Billy said.

Britain, home to 1.8 million Muslims — the majority of whom have Pakistani origins — saw two thwarted terror attacks in London and Scotland last summer, and the July 7, 2005, suicide bombings that killed 52 people on London's transit system.

Billy said recent domestic plots in the United States against military targets and airport infrastructure — as well as intelligence that showed Hezbollah operatives may be working within US borders — underscored the threat within U.S. borders.

Billy said while some cells were gaining strength in Pakistan's lawless tribal areas, officials needed to look beyond Pakistan to areas such as North and East Africa where groups and individuals were finding fresh sanctuaries.

There was no intelligence to suggest terror groups were inching closer to developing biological, chemical or nuclear capabilities, Billy said.

"Getting to that point requires an infrastructure and investment," he said.

Billy painted a picture of counterterrorism operations of the future where involvement between the military, law enforcement and intelligence officials will be crucial.

"The old dichotomy of law enforcement and intelligence — and law enforcement and military — no longer applies," Billy said. "Combatting terrorism requires a combination of all these resources and not just within our borders."

But gaps remained in international law to address issues unseen before Sept. 11, he said.

When the U.S. prison camp in Guantanamo Bay opened in 2002, the FBI was one of the first to warn the Pentagon of interrogation techniques that could make it difficult to prosecute terror suspects because the evidence could be ruled inadmissible.

Only about a dozen or so of the hundreds of Guantanamo detainees have been charged.