Law enforcement sources said agencies in the New York area were concerned about the extraditions - which could occur any day - and plans are already being made to bolster security, if necessary, to cope with related threats. The sources spoke on condition of anonymity.
While the defendants are likely to make a brief court appearance soon after they arrive on U.S. soil, there is little likelihood that a full trial will begin anytime soon. More likely, an official said, is extended pre-trial litigation by the defendants.
In anticipation of possible political attacks, administration officials are strongly defending the guarantees the United States gave to Britain promising that the extradited men would be tried in civilian courts.
Those pledges, a U.S. official said, were given over a period of years by both Obama and his predecessor, President George W. Bush.
One of the arguments on which the defendants based years of appeals against extradition was that they could be sent to Guantanamo, where they could be tried by military tribunals and sentenced to death.
But "the British government requested, and the U.S. government provided, binding commitments that, if extradited, the defendants would only be tried in a federal civilian court, rather than a military commission," a U.S. counterterrorism official said. The United States also promised that no death sentences would be sought.
Without those commitments, extradition would not have been possible, the official said, and because the five face no charges in Britain they likely would have been released.