A statement issued by Chertoff said "currently, there is no indication ... of plotting within the United States."
A U.S. law enforcement official said there have been no arrests in the United States connected to the plot.
A senior U.S. counterterrorism official said authorities believe dozens of people possibly as many as 50 were involved in the overseas plot that was uncovered around 10 p.m. EST Wednesday. The plan "had a footprint to al-Qaida back to it," said the official, speaking on condition of anonymity because of the sensitivity of the situation.
The plan involved airline passengers hiding masked explosives in carryon luggage, the official said. "They were not yet sitting on an airplane," but were very close to traveling, the official said, calling the plot "the real deal."
Another Western counterterrorism official, also speaking on condition of anonymity about the highly sensitive investigation, called the investigation "very serious."
U.S. intelligence, particularly the CIA, has been working closely with the Brits on the investigation, which has been ongoing for months, the second official said.
Authorities have not yet arrested or detained all suspects who are believed to be involved in the plot, the official said, prompting Chertoff's alarm.