WASHINGTON â€"  A year after the Navy SEAL raid that killed Usama bin Laden, the Al Qaeda that carried out the Sept. 11 attacks is essentially gone but its affiliates remain a threat to America, U.S. counterterrorist officials say.

Core Al Qaeda's new leader, Ayman al-Zawahri, still aspires to attack the U.S., but his Pakistan-based group is scrambling to survive, under fire from CIA drone strikes and lying low for fear of another U.S. raid. That has lessened the threat of another complex attack like a nuclear dirty bomb or a biological weapon, the officials say.

Al Qaeda's loyal offshoots are still dangerous, especially Yemen's Al Qaeda in the Arabian Peninsula, or AQAP. While not yet able to carry out complex attacks inside the U.S., such groups are capable of hitting Western targets overseas and are building armies and expertise while plotting violence, according to senior U.S. counterterrorist officials who briefed reporters Friday.

"Each will seek opportunities to strike Western interests in its operating area, but each group will have different intent and ability to execute those plans," said Robert Cardillo, a deputy director at the Office of the Director of National Intelligence. The other officials were authorized to speak only on condition of anonymity.

The shift from a single, deadly group to a more amorphous threat may not seem much of an improvement. But the U.S. believes that the bin Laden raid and continued U.S. counterterrorist action have reduced the chance of a sophisticated, multipronged attack on the U.S. like the attacks of Sept. 11 or the deadly bombings in Madrid in 2004 and in London in 2005.

An attack with weapons of mass destruction -- chemical, biological or nuclear -- by any Al Qaeda-related terror group also seems less likely in the coming year, Cardillo said.

Al Qaeda's Zawahri has not managed to harness multiple groups into a cohesive force focused on a single, catastrophic attack, officials said.

Al Qaeda's key affiliates in Yemen, Somalia, Iraq and North Africa have pledged allegiance to Zawahri but, unimpressed with his leadership, "have not offered the deference they gave bin Laden," Cardillo said. Zawahri has a reputation as an abrasive manager and a less than charismatic speaker.

That loss of a single, charismatic voice likely means "multiple voices will provide inspiration for the movement," leading to a bout of soul-searching as to what the splinter groups want to target and why, Cardillo said.

"There will be a vigorous debate about local versus global jihad within and among terror organizations," he said.

Another potentially positive sign is Al Qaeda's failure to hijack the Arab Spring revolt in Egypt, Tunisia or Libya. On the negative side, the officials said, Al Qaeda is working hard to co-opt rebels in Syria.

If the political wrangling in any of the post-revolt nations fails to produce stable, responsive governments, Al Qaeda and its ilk may be able to seize the void, the officials said.

That's what has occurred in Yemen, where AQAP has taken full advantage of the local government's preoccupation fighting multiple political opponents. AQAP has grown in size and territory covered despite constant and expanded targeting by Yemeni and U.S. counterterrorist forces, the officials said.

Another threat they cited: Homegrown extremists, either lone actors or small groups inspired by Al Qaeda, who remain intent on committing violence.

The officials also noted that every time U.S. counterterrorist forces strike, they must take care to avoid everything from civilian casualties to hitting the wrong target, lest the blowback produce more enemies.

"The key challenge will be balancing aggressive counterterrorism operations, with the risk of exacerbating the anti-Western global agenda" of Al Qaeda and its affiliates, Cardillo said.