"But obviously the investigation continues. And we have instituted more screening and what we call mitigation measures at airports."
Napolitano also confirmed that there had been no US air marshal on Abdulmutallab's flight.
The armed, plain clothes officials are not on every flight.
Her comments came as another man, also a Nigerian national, was briefly detained by US authorities after triggering a security scare on the same Detroit-bound flight involved in Friday's incident.
Officials say the man had spent an unduly long time in the plane's lavatory, raising concern among the crew.
But the FBI who questioned the man said later he had apparently become ill mid-flight, and upon investigation, the incident had been deemed "non-serious."
Nonetheless, the alert highlighted the continued anxiety generated by Friday's apparent attempted bombing.
Barack Obama, the US president, has ordered a review of air security wanting to know how a man carrying explosives had managed to board a flight from Amsterdam to Detroit.
An initial FBI analysis found the device used by Abdulmutallab contained PETN, also known as pentaerythritol, one of the explosives carried by the so-called "shoe bomber" Richard Reid in his failed attempt to blow up a US airliner just before Christmas in 2001.
|US airports have stepped up security in the wake of Sunday's incident [AFP]
Among the new security measures introduced in the wake of Friday's incident, passengers arriving in the US said they had been prevented from leaving their seats during the final hour of the flight and had access to hand-luggage restricted.
Meanwhile Abdulmutallab himself is due to make his first appearance in federal court in Detroit on Monday.
He was charged on Saturday at the University of Michigan Medical Centre in Ann Arbor, where he was being treated for burns apparently caused when he tried to detonate the explosives.
According to US officials Abdulmutallab, the son of a prominent Nigerian family, was named on the US Terrorist Identities Datamart Environment (TIDE) security watch list, a document which includes about 550,000 people.
However, the Nigerian was not on the smaller Terrorist Screening Data Base (TSDB) and was not flagged for mandatory extra airport security screening or included on the "no fly" list.
'Specific and credible'
Speaking to CNN, Janet Napolitano, the US homeland security chief, said there had not been enough negative data to add him to that list.
The government needs "information that's specific and credible if you're actually going to bar someone", she said.
|Abdulmutallab is the son of a prominent Nigerian banker [Reuters]
US authorities told The Associated Press news agency Abdulmutallab came to the attention of intelligence officials in November when his father went to the US embassy in Abuja to express concerns about his son.
Al Jazeera's Yvonne Ndege, reporting from the Nigerian capital, said authorities there had launched an investigation, concentrating on how the suspect breached security and boarded the aircraft.
He is believed to have started his journey in Lagos before the flight stopped at Amsterdam's Schiphol airport on the way to Detroit.
Ndege said that Abdulmutallab's family had been called to the capital for questioning, but that there was no suggestion that they were involved in illegal activities.
"There's been shock, condemnation and disbelieve from millions of Nigerians ... They simply cannot believe that the son of a prominent banker ... from a very privileged family, somebody who was educated privately in some of the world's best educational institution could be involved in this kind of event."