Speaking on ABC's "This Week," Brennan, President Barack Obama's top adviser on combating terrorism, said, "I think the sophistication of these IEDs shows, at least in my mind, that it was an al-Qaida in the Arabian Peninsula effort, and that there are a number of people that are involved in this. And so it's not just these two individuals. We're looking for a lot more."
An engineering student and her mother have been arrested in Yemen.
On "Fox News Sunday" Brennan said, "We cannot presume that there are none other that are out there."
On CBS's "Face the Nation," he added, "Clearly, the sophistication of this device requires I think a number of people who are involved. We are concerned about individuals in Yemen who have very proficient bomb-making capabilities who are very dangerous."
Brennan told ABC host Christiane Amanpour that Saudi authorities tipped off the United States about the scheme.
"We owe a debt of gratitude to the Saudis. I think their actions really saved lives here."
He also told CNN's "State of the Union": "These devices did not need someone to actually physically detonate them. … They could have been detonated in the location where they were on the plane or they could have been detonated when they reached their destination."
But he added, "We feel as though we've stopped all of the packages that have come into the United States that were originating in Yemen."
Brennan told NBC's "Meet the Press" host David Gregory that "FedEx and UPS have stopped all the packages that are coming into the United States that were being mailed from Yemen."