Lieberman, chairman of the Senate homeland security committee, called the attempted attack a "break through our defenses," comparing it to the Fort Hood shooting last year and the failed Christmas Day airline attack.
"We were lucky. We did not prevent the attempted attack," Lieberman, I-Conn., told "Fox News Sunday." "It's hard to stop them every time, but that has to be our goal. ... So I'd say in terms of prevention, the system failed."
The car bomb in Times Square last Saturday failed to properly detonate, giving police time to evacuate the crowded downtown hub before hauling the explosive-laden vehicle away. Shahzad, who is of Pakistani descent, became a U.S. citizen a year before he allegedly received training and constructed the crude car bomb.
"We're not lucky. We're good," he told "Fox News Sunday." "We've stopped many, many operations, terrorist plots, from taking place here in the United States and overseas. On a daily basis, we are being successful at finding them, arresting them, killing them."
Still, he said, officials will learn lessons and "refine our system as needed."