US Steps Up Security Following Attack in Britain (2/2)

He added that there will be what he described as "enhanced air marshal work" on trans-Atlantic flights, with special attention to Britain. U.S. air marshals have been accompanying U.S.-bound flights since a plot to blow up airliners between the United States and Britain was uncovered last August in London.

Chertoff said Washington is increasingly concerned about people with European citizenship going to South Asia to get terrorism training, and then returning to carry out attacks in Europe or the United States. He told CNN there are, what he described as, a "significant number of people" within the United States who have suspected links to terrorism.

"I am not saying those are linked to al Qaida, but they certainly indicate the possibility of people becoming radicalized and deciding they want to carry out attacks on their own. So, this is not, by no means, a European problem, but a problem here as well."

Chertoff said he has no immediate plans to raise the overall threat level in the United States, which is at yellow, signifying an elevated level. Since last August, the U.S. threat level for all domestic and international flights has been more serious, at level orange.