(CNN) -- Homeland Security Secretary Janet Napolitano said Sunday there was no indication so far that an alleged botched terror attack on a U.S. airliner was part of a broad international effort.
"Right now we have no indication that it is part of anything larger," Napolitano told CNN's "State of the Union" program.
A lone suspect is accused of trying to ignite an explosive device on a Northwest Airlines flight from Amsterdam, The Netherlands, to Detroit, Michigan, on Christmas Day.
The device apparently failed to detonate and the suspect was subdued.
Napolitano said the suspect, Umar Farouk Abdulmutallab, was on a broad security watch list that contains "half a million" names and is shared with airlines and foreign security agencies. However, a lack of specific evidence prevented him from being classified as a greater security risk that would bar him from travel to the United States, she said.
"You need information that is specific and credible if you're going to bar people from air travel," Napolitano said.
Despite a possible tragedy, Napolitano said the response system worked as passengers and crew avoided panic and subdued the suspect while authorities notified other flights in the air and kept the air transport system operating.
"The whole process of making sure that we respond properly, correctly and effectively went very smoothly," Napolitano said.
Security screening has been increased since the incident, with different procedures at different airports to prevent predictability, Napolitano said.
"While we continue to investigate the source of this incident, the traveling public should be very confident of what we're doing now," she said.