The declassified key findings, to be released publicly on Tuesday, were obtained in advance by The Associated Press.
The report lays out a range of dangers — from al Qaeda to Lebanese Hezbollah to non-Muslim radical groups — that pose a "persistent and evolving threat" to the country over the next three years. As expected, however, the findings focus most of their attention on the gravest terror problem: Osama bin Laden's al Qaeda network.
Homeland Security Secretary Michael Chertoff tells CBS Evening News anchor Katie Couric in an exclusive interview that "what this intelligence estimate confirms is a general sense that we're entering a period where we do have to be concerned about a heightened risk. There is a recognition that we've degraded the capabilities of al Qaeda, but also we have to be concerned that they are beginning to regenerate and that they have found some kind of safe haven in certain parts of South Asia."
Fran Townsend, the White House's Homeland Security adviser, said at a Tuesday morning press conference, "We are facing a persistent terrorist enemy led by al Qaeda that remains driven and intent on attacking the homeland and that continues to adapt and improve its capabilities."
She added that "we have no credible information pointing to a specific imminent attack or the timing or execution of such an attack. But the warning is clear, and we are taking it seriously."
Officials say there's no evidence al Qaeda has acquired weapons of mass destruction or injected a sleeper cell into the U.S. but both goals remain top priorities, reports CBS News correspondent Bob Orr.
"Of note," the report said, "we assess that al Qaeda will probably seek to leverage the contacts and capabilities of al Qaeda in Iraq (AQI), its most visible and capable affiliate and the only one known to have expressed a desire to attack the homeland."
The analysts also found that al Qaeda's association with its Iraqi affiliate helps the group to energize the broader Sunni Muslim extremist community, raise resources and recruit and indoctrinate operatives — "including for homeland attacks."
It also paints a worrisome picture of increased training at camps along the Pakistan-Afghanistan border, giving al Qaeda added capability to mount strikes on the U.S. and its key allies.
CBS News correspondent Elizabeth Palmer reported that while the northern area of Pakistan, much of which is controlled by local tribes, has always been a stronghold of the Taliban, it's now also .
The report said that the radical and violent segment of the Muslim population in the West is expanding.
"We assess that this internal Muslim terrorist threat is not likely to be as severe as it is in Europe, however," the report said.
Orr adds that there is some good news in the otherwise grim assessment: according to the report, counterterrorism efforts and new security measures have "constrained the ability" of al Qaeda "to attack the U.S. Homeland again." And, terrorists perceive the United States to be "a harder target to strike than on 9/11."
National Intelligence Estimates are the most authoritative written judgments of the 16 spy agencies across the breadth of the U.S. government. These agencies reflect the consensus, long-term thinking of top intelligence analysts. Portions of the documents are occasionally declassified for public release.
"We don't keep it on the shelf and say `Let's look for a convenient time,'" Snow said.
"(What) we're trying to remind people is that this is a real threat. This is not an attempt to divert. As a matter of fact ... we would much rather — one of the things we'd like to do is call attention to the successes in the field" in Iraq, he said.
Democrats said the report was proof U.S. anti-terrorism efforts were being drained by the Iraq war.
"We must responsibly redeploy our troops out of Iraq, handing responsibility for security over to the Iraqis and leaving only those forces required for limited missions," said Rep. Ike Skelton, chairman of the House Armed Services Committee. "This will allow us to concentrate our efforts on Afghanistan and the al Qaeda terrorists who attacked us on 9/11."
House Republican leader Rep. John Boehner said the report confirms gains made by Mr. Bush and blamed Democrats for being too soft on terrorism.
"Retreat is not a new way forward when the safety and security of future generations of Americans are at stake," he said in a statement.
The new report echoed statements made by senior intelligence officials over the last year, including the assessment of spy agencies that the United States is in a "heightened threat environment." It also provided new details on their thinking and concerns.
For instance, the report says that worldwide counterterrorism efforts since 2001 have constrained al Qaeda's ability to attack the U.S. again and convinced terror groups that U.S. soil is a tougher target.
But, the report quickly adds, analysts are concerned "that this level of international cooperation may wane as 9/11 becomes a more distant memory and perceptions of the threat diverge."
Among the report's other findings: