In his first public appearance as director of the CIA, Porter Goss gave a chilling assessment of the dangers posed by nuclear material that is missing from nuclear storage sites in Russia.
Goss and FBI Director Robert Mueller testified before the Senate Select Committee on Intelligence about the international threats against the United States.
Responding to a question from Sen. Jay Rockefeller, D-W.Va., about materials missing from Russian nuclear facilities, Goss said: "There is sufficient material unaccounted for, so that it would be possible for those with know-how to construct a nuclear weapon."
Goss said he could not assure the American people that the missing nuclear material had not found its way into terrorists' hands.
A former top official at the Department of Energy told ABC News that Goss's statement understated the threat. There could be enough missing material in the Russian inventory to make hundreds or thousands of nuclear weapons, but no one -- neither the Russians nor Western intelligence agencies -- knows for sure, the former official said.
There is no way to determine the quantity of missing material in Russia, the source said, because neither the Russian government nor the Soviets before them ever adopted a "mass balance" inventory system that tracks how much nuclear material is produced and where it ends up being used. The U.S. government adopted a mass-balance inventory system in the 1960s, the source said.
Mueller also confirmed new intelligence which suggests al Qaeda is trying to acquire weapons of mass destruction.
"I am also very concerned with the growing body of sensitive reporting that continues to show al Qaeda's clear intention to obtain and ultimately use some form of chemical, biological, radiological, nuclear or high-energy explosives," he said.
During his testimony, Goss said the war in Iraq has created a new threat, which may pose an international danger for years to come.
"Those jihadists who survive will leave Iraq experienced in and focused on acts of urban terrorism," said Goss. "They represent a potential pool of contacts to build transnational terrorist cells, groups and networks."
Counterterrorism analysts believe, if history is a guide, Goss is right.
"Osama bin Laden is a result of the jihad in Afghanistan," said former FBI counterterrorism chief Harry "Skip" Brandon. "We're going to see him or people like him in years to come. They're being trained in Iraq today. I look at early Afghanistan -- a lot of people refer to it as 'test-tube jihad.' The people that did make it out of Afghanistan spread throughout the region, and they are coming back to haunt us now. We'll see that again."
Mueller said that while the FBI is monitoring suspected terrorists in the United States, he cannot rule out that there are sleeper terrorist cells in the country that have gone undetected.
"Finding them is the top priority for the FBI, but it is also one of our most difficult challenges," he said.
U.S. officials acknowledge their grim conclusions are alarming but say they are based on real intelligence they see every day. The quality of that intelligence, however, remains in question.
ABC News' Pierre Thomas filed this report for "World News Tonight," with reporting by ABC News' Howard Rosenberg.