Washington (CNN) -- Explosive devices in cargo packages addressed to Chicago, Illinois, destinations appear to have been designed to detonate on their own, without someone having to set them off, the top White House counterterrorism official told CNN on Sunday.
John Brennan, President Barack Obama's assistant for homeland security and counterterrorism, said on CNN's "State of the Union" program that the sophisticated explosives could have been intended to blow up the air cargo planes carrying them, as suspected by British authorities.
"It is my understanding that these devices did not need somebody to detonate them," Brennan said, adding that U.S. authorities continued to investigate.
Brennan confirmed the arrests of two people in Yemen in connection with the case and said investigators were checking on the possibility of U.S. connections or suspects, noting the explosive devices were addressed to Chicago locations.
However, Brennan stopped short of saying there were any known U.S. suspects so far.
Asked about the possibility of further such explosive devices on other planes heading to the United States, Brennan said authorities must operate on that assumption until proven otherwise.
"It would be very imprudent for me and for others within the counterterrorism community here and abroad to presume that there are no others out there," Brennan said, later adding: "We feel as though we have stopped all the packages that were coming to the United States that were originating in Yemen."