Security experts noted the extremists had threatened the former prime minister for seeking democratic reform and for her links to the West, CNN reported.

Pakistani President Pervez Musharraf said on state television the killers were the same extremists his military has been fighting. He vowed to bring them to justice.

Vince Cannistraro, a veteran U.S. Central Intelligence Agency official who ran the agency's counterterrorism operations from 1989 to 1991, told CNN several groups had something to gain from Bhutto's death.

"Clearly al-Qaida and Islamic fundamentalists had expressed hatred toward her," Cannistraro told CNN. "They would be No. 1 on the list."

U.S. President George W. Bush blamed "murderous extremists who are trying to undermine Pakistan's democracy." British Foreign Secretary David Miliband also blamed "extremist groups."

Bhutto, who escaped unhurt in a bomb attack in October, hours after she returned home following her long self-imposed exile, had blamed al-Qaida and the Taliban for that attack, CNN said.

"Don't forget that this was a suicide bombing," Cannistraro said. "That's a technique we have seen used by the Taliban and al-Qaida."