The head of U.S. homeland security, Michael Chertoff, says officials remain vigilant for terrorist attacks after a plot to blow up airliners was uncovered last week.
Chertoff told Fox News Sunday U.S. officials are concerned that some people involved in the alleged plot may still be at large.
He says there also is concern that other groups may try to carry out attacks because they think U.S. officials are distracted by the situation.
Chertoff added there is no evidence that planning or operational activities took place in the United States. British authorities detained 23 people last week.
U.S. officials raised the security alert level on flights from Britain to the United States after the plot was uncovered.
Chertoff says officials planned to announce new security measures for flights today.