A chemical or biological attack by al-Qaida and its offshoots remains a threat, despite the killing of terror leader Osama bin Laden, top former U.S. counterterrorist officials said Thursday.

"We still have pockets of al-Qaida around the world who see this as a key way to fight us," especially the offshoot in Yemen, Mike Leiter, the just-retired director of the National Counterterrorism Center, told an audience at the Aspen Security Forum. "The potential threat from al-Qaida in the Arabian Peninsula is very real."

Leiter said the new breed of terrorists understands that killing a few Americans can cause as much fear as the massive plots bin Laden backed.

"The most likely ... are simple forms of chemical or biological weapons" rather than a nuclear attack, Leiter said, using the poison ricin as an example of something that's easily made.

"Is it going to kill many people? No. Is it going to scare people? Yes," he said.

"Bin Laden was really prioritizing the big attack," Leiter added. "Some of them may have fantasies about pulling off another 9/11," but his affiliates realize they can affect U.S. strategy and society with smaller scale attacks.

"Anwar al-Awlaki gets that," Leiter said, naming the U.S.-born radical cleric of the Yemeni branch. And so do other offshoots, like the Pakistani Taliban, which sent a bomber to try to blow up a car in the middle of Times Square a year ago, he said.

With bin Laden gone, Leiter and former CIA Deputy Director John McLaughlin both predicted new leader Ayman al-Zawahiri launch similar smaller scale campaign.

"Zawahiri will probably favor smaller targets," McLaughlin told the audience. "Bin Laden did not."

Leiter and McLaughlin both said Zawahiri's core al-Qaida was weaker after bin Laden's killing.

"I think it is now possible ... to actually visualize, to imagine its collapse," McLaughlin said, speaking of the original core group. But he warned against underestimating Zawahiri or his followers.

"He's not as charismatic ... but he may be more disciplined, he said, adding that Zawahiri is a physician who has long been interested in using weapons of mass destruction to attack.

"What we will know is there has been no successful inbound attack since 9/11 that we can attribute directly to al-Qaida," added Charles Allen, who has held multiple top positions at the CIA over the years. "But we can see this metastasized network linked by Internet that is self-sustaining across the world."

Leiter said the trove of information, including inter-al-Qaida communications, taken from bin Laden's compound where he was killed by U.S. commandoes showed the group already was struggling.

He said the documents revealed bin Laden was not the CEO of a large multinational corporation, as analysts thought, but the "slightly out of touch coordinator of a broad dysfunctional family who, frankly, were operating on their own agendas more than his."

But he said al-Qaida and the other groups still have enough organization and staff to keep attacking.

Leiter warned that intelligence and military leaders had to figure out how to keep their staff members, who joined after Sept. 11 to track and fight terrorists in war zones, from getting bored and leaving, because while the U.S. may be drawing down its military forces in Iraq and Afghanistan, the anti-terror war is far from over.

"Smaller scale terrorist attacks are with us for at least the foreseeable future," Leiter said.

"This is going to happen," Allen added.