But she said Monday her comments were in reference to security after the incident occurred, and acknowledged that more needs to be done to address the discrepancies in the various government lists.
A DHS spokesperson told ABC News that Napolitano spoke earlier Monday to Dutch Justice Minister Ernst Hirsch Ballin, who conveyed to her that they are now using advanced imaging technology to screen U.S.-bound passengers at the Schiphol Airport, where Abdulmutallab departed for his flight to Michigan.
Abdulmutallab was on the U.S. government's terror-watch list but not on the no-fly list, even though the United Kingdom had denied him a visa and his father had alerted the U.S. Embassy in Nigeria of his son's increasing radicalization.
"Meaningless, to be quite honest," security analyst Douglas R. Laird said of Napolitano's assurances. "They don't have the technology in place that they need to do the job properly."
Even some Democrats said Napolitano dropped the ball.
"The secretary of Homeland Security put the more pressure on the president by making those statements on Sunday where she said the system did work, and obviously she's trying to roll those back," Dowd said.
Some editorialists have compared Napolitano's remarks to former President George W. Bush's praising then-FEMA Director Michael Brown for doing "a heck of a job" after Hurricane Katrina.
"I would have a hard time arguing it's the same quality," Dowd said on "GMA." "It is a problem where we have now been in this situation for eight years, and we have a person that is on a watch list -- or is on some sort of a list, who buys a ticket with cash, who the British refused to let ... in the country ... and get on an airplane, and only through his incompetence, not anything we did ... the plane didn't go down and that's the problem."
"There needs to be a real, real investigation here," Carville said. "If we get answers and we know people are working to correct it, people will feel better."
Despite the president's attempt to reassure the public, some Republicans are seizing on the plot to criticize the administration. In an e-mail solicitation Monday, Hoesktra attacked the Obama administration and then asked for monetary support for his gubernatorial campaign.
"Since President Obama took office, he and his left-wing cronies have taken steps to undermine the work of our brave men and women who work tirelessly to keep us safe," the e-mail said. "Barack Obama's policies may impress the 'blame America first' crowd at home and his thousands of fans overseas, but they sure don't do anything to protect our families in Michigan or the rest of America.
"I will be a governor who fights, every day, to keep Michigan safe."
Carville assailed Hoekstra for using politics to raise money for his campaign.
"I think this is the same gentleman, people need to know, that had a press conference and said we found weapons of mass destruction in Iraq," Carville said. "I think this man has some sort of intellectual challenges. ... And it's kind of odd that you would put it in a fundraising letter."
ABC News' Yunji de Nies and Kirit Radia contributed to this report.