This story was written for CBSNews.com by Pakistan-based journalist Farhan Bokhari and CBS News correspondent Sheila MacVicar.
Intelligence agencies have been warned that al Qaeda may be planning to attack air and rail travel in Europe in actions that may occur during the busy holiday travel season, CBS News has learned exclusively.
In separate interviews with Arab and other intelligence sources, CBS News has been told that the warnings come from interrogations of al Qaeda suspects who recently left Afghanistan and Pakistan.
"One suspect said plans for repeating the Heathrow attempt (a reference to the failed 'liquid bomb' plot interrupted in August) were all prepared. It is now a matter of taking action," said one Arab official who asked not to be named because he is not authorized to speak to the media. "Al Qaeda's strategy appears to be raising the pressure in Europe."
In a move that has been puzzling intelligence agencies, al Qaeda has been withdrawing well-trained Arab fighters from the mountains and battlefields of Afghanistan.
In detailed interviews with Arab diplomats, intelligence sources and Pakistani and western officials, Arab members have been leaving Afghanistan for the past six months while handing over its militant activities in Afghanistan to that country's resurgent Taliban movement.
The new information helps to shed fresh light on a key mystery at the heart of al Qaeda's decision, first reported by CBS News in September, to withdraw its Arab members, fighters and logisticians from Afghanistan.
Rohan Gunaratna, head of terrorism research at Singapore's Institute for Defense and Strategic Studies, and the author of "Inside Al Qaeda; Global Network of Terror," said "We have seen that several hundred, perhaps five to six hundred al Qaeda members who were located on the Afghan-Pakistan border, have now left."
an exceprt of Katie Couric's interview with counterterrorism expert John Brennan about CBS News' report.
Gunaratna adds that these al Qaeda members have returned "to their own home countries, particularly to Iraq, and also to other Middle Eastern countries such as Saudi Arabia and Yemen. We have seen a shift in Al Qaeda's thinking, in strategy."
Some of those who returned to their native countries in the Arab world and were caught have told investigators that the orders only specified al Qaeda's decision for the Arabs to depart Afghanistan.
The choice of a route was left to al Qaeda's local commanders in Afghanistan, according to information given by arrested suspects to their interrogators.
There is no consensus on the approximate numbers of Arabs who remain in Afghanistan, though Arab diplomats with responsibilities of tracking al Qaeda said, they suspect the number of active Arab fighters to run in the several hundreds in spring this year.
"Now, maybe there are a couple of hundred people left in Afghanistan, maybe a bit more or a bit less," said one unnamed Arab diplomat, who says it was "impossible to tell the numbers accurately. But there's enough information to say that al Qaeda's Arab component in Afghanistan is very scaled down."
The first indication of Afghan-based al Qaeda Arabs beginning to leave for Iraq came almost two years ago in deep background briefings given by Pakistani intelligence officials selectively to senior journalists. However, the movement had been long considered voluntary and more an act of choice by individual fighters or small groups of militants than a decision by Al Qaeda's high command.
A senior Arab diplomat said intelligence officers in his country thoroughly interrogated a group of men in their 20s and 30s who were caught earlier this summer after they returned from Afghanistan.
"The standard answer was: We don't know why we were told to leave. The orders were very specific — leave Afghanistan now without wasting much time." The diplomat spoke on the condition that neither his identity nor that of his country would be revealed.
Intelligence analysts and security sources say one reason why al Qaeda might feel confident in leaving the battlefield in Afghanistan largely in the hands of the Afghan Taliban is that the Taliban have shown new skill and ferocity in fighting U.S. and coalition forces.
"As you can see, the increasing ferocity of these attacks has put the spotlight more on the Taliban," said one western diplomat. "When we speak of Afghanistan these days, we talk mostly of the Taliban. In the Afghan context, al Qaeda is now talked about infrequently. Does this not tell you something about a trend, that maybe al Qaeda as we saw it once being led by its Arab fighters in Afghanistan, has become less of a reality as time goes by?" he asked.
"There is very little al Qaeda Arabs can do today in the Afghanistan/Pakistan theater," said Gunaratna. "Firstly, they will be spotted and they will be killed or captured. Second, today in Afghanistan Arabs are not really required to fight the coalition and U.S. forces, because the Taliban are very well trained and very well motivated."
Al Qaeda Arabs did provide suicide bombers — until now, a role Afghan Taliban were unwilling to undertake. In recent months, however, as Taliban propaganda videos have shown, Afghans have signed wills and undertaken suicide missions.
"There are two ways of looking at what is going on between the Taliban and al Qaeda," said a Pakistani security official who asked not to be named. "Either it's the case that the Taliban have become so strong by virtue of sharply rising ethnic 'Pushtoon' support to their cause that al Qaeda is overshadowed and wants to leave Afghanistan.
"Or it's a matter that al Qaeda sees its work for certain being carried forward by the Taliban and has therefore decided to leave. Whatever the motive, the fact is, al Qaeda's Arab wings seem to be leaving," the Pakistani official said.
"We believe that today the Arabs are much more useful to bin Laden if they are deployed in their home countries, particularly in the Middle East, to target Western assets, Western interests. Because of that we have seen a shift in al Qaeda thinking and al Qaeda strategy in encouraging members to leave Afghanistan and Pakistan," says Gunaratna.
Arab intelligence sources tell CBS News the militants have left Afghanistan by routes taking them through the former Soviet Republic, Pakistan and Dubai.
Sources confirmed that some of the militants traveled through Iran. But they also warned that it would be an exaggeration to state that Iran had become the main transit point for al Qaeda militants leaving Afghanistan on their return journey to the Arab world.
"We have picked intelligence information suggesting that Iran's Revolutionary Guards had helped some of these Arabs to travel to Iraq. But it would be wrong to say that Iran has become the main route for this," said another Arab diplomat who also spoke on the condition of anonymity.
The diplomat said he believed al Qaeda suspects were also attempting to undertake return journeys through the former Soviet central Asian republics or via Pakistan. "I doubt if al Qaeda would put all of its eggs in one basket — one country. Why would they do that? These people are not stupid. They would never undertake such an operation through just one country," he said.
Iran has acknowledged to the United States it has a number of key al Qaeda figures under "house arrest," including at least one of bin Laden's sons — the suspects have been in Iran since the fall of Tora Bora in 2001.
There is disagreement between security sources and analysts about the amount of freedom those in Iranian custody enjoy and why Iran continues to give them shelter.
Intelligence sources tell CBS News that none of those interrogated upon returning to their home country has provided any specific information about al Qaeda's future plans.
Michael Scheuer, head of the CIA's bin Laden unit until 1999 and now a CBS News consultant on terrorism, said, "it appears now that al Qaeda is sending some of those trained fighters back to increase the pace of the insurgency, some to Iraq and some to the Arabian peninsula."
"It clearly means the security services in the Gulf, on the Arabian peninsula, will be tested by much more experienced people than they're fighting at the moment."
Analysts point to statements in recent months issued by Ayman al Zawahiri, Al Qaeda's number two, calling for attacks on Middle East oil installations.
"What they want to do is to hit something that will affect the Americans very markedly, and they've determined that the oil production facilities, whether it's pipelines or refineries, are something that can really contribute to the war against the United States," Scheuer said.
"It would be a win-win situation. It's taken al Qaeda the better part of a decade to figure out how to attack oil targets without hurting Muslims to any great extent and I think they finally stumbled on to it. The attack early this year on Abqaiq (the world's largest oil production facility in Saudi Arabia) was a failure, but it immediately drove the price of oil up more than $2 a barrel," he said.
Western diplomats said the U.S. administration so far had paid little attention to the movement of Arab fighters, as its bigger interest for now remains the challenge of tackling the Taliban.
By Farhan Bokhari and Sheila MacVicar