An administration official and a military adviser to Iraqi commanders attribute the decline to a fairly new phenomenon: Al Qaeda's call for mass killings in the name of Islam is losing some of its appeal with young Arabs in North Africa and Saudi Arabia, where most of the bombers originate.
The decline also parallels the battlefield losses al Qaeda has suffered in the past 12 months in Iraq's Anbar province and the greater Baghdad region. This has made it more difficult for al Qaeda in Iraq to facilitate the secret movement of foreigners from the Syrian border to safe houses where they are trained and assigned a target.
"There has been a sharp decline in the amount of suicide bombers coming into Iraq," said a senior intelligence official who spoke on the condition of anonymity. "It's harder for suicide bombers to get into the country. The al Qaeda in Iraq is a shadow of what it once was. And Iraq is a more hostile area for suicide bombers to operate."
The senior official said al Qaeda suicide attacks averaged 50 per month last year, but are as low as 20 a month now.
"They are still cause for concern," the official said.
According to the latest Pentagon report on stability in Iraq, the number of all attacks in Iraq dropped from 1,500 a week in February 2007 to 450 a week in February 2008.
Last year, U.S. military officials say, Sunni Arab Iraqis in large numbers began rejecting al Qaeda's harsh ways and started aiding allied troops in ridding terrorists from their neighborhoods.
A Bush administration official who monitors Iraq confirmed the sharp decline. This official depicted it as a long-running trend that began last year and continues today, rather than just brief dip. "There is no question there are fewer suicide bombers inside Iraq," the official said.
A military intelligence officer previously has told The Washington Times that interrogations of captured foreign fighters showed that most bombers came from Saudi Arabia and North Africa.
This source said al Qaeda operated three main routes: ferrying recruits from Syria into Mosul in the north, into Baghdad in the heart of the country, and into Anbar province in the west.
The senior intelligence official said that "the al Qaeda in Iraq network that would have supported bringing these people in, [is] a shadow of what it once was. They lost a lot of their key people."
Perhaps the most telling blow was the U.S. air strike in 2006 that killed Abu Musab Zarqawi, the founder of al Qaeda in Iraq whose persona in the Arab world helped coax disaffected youth to heed his call for jihad and martyrdom.
Today, the U.S. thinks the last al Qaeda bastion is greater Mosul, where Iraqi and American troops have been conducting counterterrorism strikes since the winter.
"I think they have been operationally defeated since the end of 2007," said retired Army Gen. Jack Keane, an adviser to top commanders in Iraq who toured the country in March.
"The command doesn't want to say that, but they cannot muster complex operations like they used to do in the past, nor sustain a level of operations like they have done in the past," he said. "The effort in the north is to bring this to a culmination. I call it, 'finish the al Qaeda.' "
However, he added that being a terrorist organization, al Qaeda "will never go completely away."
In the current edition of the New Republic magazine, terrorism experts Paul Cruickshank and Peter Bergen, who has written a biography of al Qaeda leader Osama bin Laden, document what they think is a pronounced shift by Muslims away from al Qaeda.
"Why have clerics and militants once considered allies by Al Qaeda's leaders turned against them?" the two writers ask.
"To a large extent, it is because Al Qaeda and its affiliates have increasingly adopted the doctrine of takfir, by which they claim the right to decide who is a 'true' Muslim," they write.
The authors note that al Qaeda's suicide bombers have killed more than 10,000 Iraqis, most of them targeted simply for being Shi'ite.
"Recently, Al Qaeda in Iraq has turned its fire on Sunnis who oppose its diktats, a fact not lost on the Islamic world's Sunni majority," the authors wrote.
They argue that a significant event in the burgeoning anti-al Qaeda movement was the defection last year of Noman Benotman. A Libyan Muslim extremist, Benotman once worked to overthrow secular Arab governments, but now seeks peace in his home country.
In November, he sent a public letter to Ayman al-Zawahri calling on the al Qaeda No. 2 man to end terrorism operations around the world.
In raids this year, coalition forces have discovered kidnapped pre-teenage Iraqis being programed by al Qaeda for suicide bombings - perhaps a sign that it is more difficult to recruit foreigners.
"Foreign suicide bombers have fallen off in 2008 compared to 2007 rather significantly," Gen. Keane said. "Motivation to come to Iraq is down in the Sunnis Arab states because many believe the [al Qaeda] operation in Iraq is a lost cause. Also it is well known that it is far more likely they will not accomplish the mission because [al Qaeda's] capacity to receive them and protect them is diminished greatly."