Mr. Bush said the government had an obligation to tell Americans about the threats, even though some have questioned whether the warnings were politically motivated to strengthen the president's image as commander in chief in an election year.
"When we find out intelligence that is real, that threatens people, I believe we have an obligation as government to share that with people," Mr. Bush told a convention of minority journalists. "Imagine what would happen if we didn't share that information with the people in those buildings and something were to happen, then what would you write? What would you say?"
On Sunday, authorities elevated alert levels in New York, Washington and Newark, N.J., on the belief that terrorists might be plotting attacks on the Citigroup Center Building and New York Stock Exchange, the Prudential Building in Newark, and the International Monetary Fund and World Bank in Washington.
The intelligence behind the warnings — including hundreds of detailed surveillance photos, sketches and written documents — came from sources including a seized laptop and computer discs and from interviews after the mid-July arrest of a young Pakistani computer engineer, Mohammad Naeem Noor Khan.
Much of the information was at least several years old, some of it preceding the Sept. 11, 2001, attacks, U.S. officials have acknowledged, though some of it may have been updated as recently as January.
"The threats we're dealing with are real and therefore we must do everything we can to ferret out the truth and follow leads," the president said. "These recent threats, that are becoming more and more enriched, as you're finding out. There was more than one thread-line, threat-line. People are now seeing there was other reasons why we took the action we took."
In a related development, the arrest of a key al Qaeda operative in Britain this week broadened the global scope of the terrorist alert that has echoed from the Pakistani frontier to the streets of Manhattan.
At least 20 people have been detained in Pakistan in the past month, and Britain is holding 12 men.
The man suspected of authoring those surveillance documents was among the terror suspects arrested in Britain. Known as Abu Eisa al-Hindi or Abu Musa al-Hindi, he is a suspected senior al Qaeda member.
The official called al-Hindi "a key al Qaeda operative." A counterterrorism official, speaking on the condition of anonymity earlier this week, said much of the documentary evidence for the alerts, including surveillance reports, was in fluent English, indicating the author spent significant time in the West.
Al-Hindi was one of the 11 men picked up in raids Tuesday. Two others arrested in those raids have since been released, including a 25-year-old man who was freed Friday without charge.
The twelfth detainee is Babar Ahmad, whose arrest was announced Thursday and who is wanted in the United States for allegedly helping finance terrorist activity.
A federal prosecutor in Connecticut issued a warrant for Ahmad's arrest, and British police said that anti-terror officials were searching three "residential premises" and one business in southwest London on behalf of U.S. authorities.
Ahmad, 30, is accused in the United States of trying to raise funds for "acts of terrorism in Chechnya and Afghanistan" from 1998 through 2003, according to the U.S. extradition warrant. His detention was not believed to be linked to the arrest of the 12 on Tuesday.
A U.S. official said the United States expects to seek Ahmad's extradition from Britain but noted that such requests typically take years. Some of the Web sites Ahmad is accused of running to solicit money were operated out of Connecticut, the official said.
The arrests draw a link between two major sweeps against suspected al Qaeda networks in Pakistan and Britain — as well as the alerts announced Sunday in New York, New Jersey and Washington.
They also point, experts say, to a persistent risk of terrorist attack, even if the surveillance on the five U.S. buildings was largely conducted before Sept. 11, 2001.
Brian Jenkins, a terrorism expert, told CBS News Early Show anchor Julie Chen that this week's arrests supports the likelihood of another terrorist attack in the near future.
"I think it confirms what has been our operational presumption since 9/11 — that is these terrorists are determined to carry out further attacks against the United States abroad and potentially here if the opportunity arises," Jenkins said. "They have said this. They continue to plan for this, and so we have to continue our operations aimed at disrupting these attacks."
U.S. officials agree, saying the capture al Hindi does not mean the threat of an al Qaeda attack here in the U.S. has been eliminated, although there is no question a major part of Osama bin Laden's network is unraveling, reports CBS News Correspondent David Martin.
"Certainly, intelligence has improved, (and) security has been increased," Jenkins said. "More importantly, we're seeing excellent examples of cooperation among the intelligence services around the world. Intelligence is cumulative.
"As the services learn more about the terrorist enterprise we're dealing with, they are able to more effectively exploit the intelligence that comes from one arrest in order to quickly follow it up and exploit it with new arrests."
The CIA provided information that contributed to the detention of al-Hindi, as well as information that led the Pakistanis to detain Khan.