(CNN) -- Suspicion swirled around Islamic extremists Thursday as news spread that former Pakistani Prime Minister Benazir Bhutto had been assassinated.
Vehicles burn moments after an October 18 bomb attack against Benazir Bhutto's motorcade in Karachi, Pakistan.
No group immediately claimed responsibility, but security experts noted that extremists had threatened the Pakistani opposition leader, viewed as a moderate voice for democratic reform.
Pakistani President Pervez Musharraf said the killers were the same extremists his country has been battling. He vowed he would not rest until they are tracked down.
President Bush blamed "murderous extremists who are trying to undermine Pakistan's democracy" and said authorities must bring them to justice.
British Foreign Secretary David Miliband also pointed to "extremist groups."
Several groups had something to gain from Bhutto's death, said Vince Cannistraro, a 27-year CIA official who ran the agency's counterterrorism operations from 1989 to 1991. Watch as CNN's Peter Bergen looks at what groups might have wanted Bhutto dead »
"Clearly al Qaeda and Islamic fundamentalists had expressed hatred toward her," Cannistraro said. "They would be No. 1 on the list."
Bhutto returned to Pakistan on October 18 after eight years in self-imposed exile. Bombers attacked her convoy after her arrival, killing at least 140 of her supporters. Bhutto blamed al Qaeda and the Taliban for that attack.
Extremists opposed Bhutto for her links to democratic Western countries.
The method of Thursday's attack provided some initial clues, Cannistraro said.
"Don't forget that this was a suicide bombing," he said. "That's a technique we have seen used by the Taliban and al Qaeda."
Musharraf had struck a deal with Bhutto for him to retain the presidency and her to assume the prime minister's job, but that agreement appeared to crumble. In November, Bhutto criticized Musharraf for squelching democracy when he declared a state of emergency.
Regardless of who is behind the attack, many Pakistanis will suspect that Musharraf or his security forces played a role in Bhutto's death, said Christine Fair, an expert on the nuclear-powered country and senior political scientist at the Rand Corp. in Washington.
"No one's going to believe that in some measure -- either actively or passively -- that he was not involved in this," Fair said. "This is going to be his undoing."
After the bomb attack on her convoy in October, Bhutto blamed the Pakistani government for reducing security around her house.
"I began to feel the net was being tightened around me when police security outside my home in Karachi was reduced, even as I was told that other assassination plots were in the offing," she wrote in a November 4 commentary for CNN.com.
Nevertheless, Fair said she suspects Islamic extremists.
Another opposition leader, Nawaz Sharif, may have gained by Bhutto's departure from the Pakistani political stage. He and Bhutto were united, Cannistraro said, at least in their opposition to Musharraf. E-mail to a friend