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The bulletin, which an official described as an advisory rather than an alert, said that major U.S. cities are possible targets and that a "viable" bomb made of ammonium nitrate fertilizer and diesel fuel "could be concealed in standard luggage." That explosive mixture was used in the bombing of the federal building in Oklahoma City in 1995.

FBI spokesman Paul Bresson said the notice, which was issued late Thursday by his agency and the Homeland Security Department, was based in part on "bits and pieces" of uncorroborated intelligence and "out of an abundance of caution" following last month's train bombings in Madrid.

"We have bits of information here, and we're just passing it along," he said.

As in previous bulletins, the FBI recommended tightening security around parking lots, limiting access points, improving lighting and removing trash bins, among other measures.

Between 1997 and 2000, more than 195 terror attacks occurred on transit systems worldwide, according to congressional investigators.

More than 9 billion trips are taken each year on the U.S. public transportation system, with 32 million trips every weekday -- about 16 times the number of trips taken on airlines, according to the American Public Transit Association.

According to The Associated Press, the association estimates that $6 billion is needed to upgrade and modernize U.S. transit systems to meet security needs. The Transportation Security Administration dedicated only $10 million for passenger rail and public transit security in the current year's budget, according to the House Homeland Security Committee.

In Spain, Interior Minister Angel Acebes said experts dismantled a bomb found Friday under the tracks of Spain's bullet train line between Madrid and Seville. Acebes said it was not clear who hid the bomb, which failed to detonate because it wasn't properly connected.

On March 11, 10 backpack bombs ripped through four commuter trains in Madrid, killing 191 people and injuring more than 1,800. The focus of the investigation is a Moroccan extremist group with links to al-Qaida.

In another terrorism-related development, administration officials said Friday that starting this fall, visitors from some of the closest U.S. allies will have to be photographed and fingerprinted when they enter the United States.

Travelers from 27 countries, mostly in Europe, who can visit the United States without visas will nevertheless be processed at airports and seaports beginning Sept. 30 because most of their governments are unable to meet the October deadline for issuing high-tech "biometric" passports, the officials said.

Even travelers from Australia, which says it can meet the deadline for issuing the passports which include fingerprint and iris identification features that make them almost impossible to counterfeit, will undergo the 23-second process when they arrive.

U.S. officials said the processing requirement for the so-called "visa waiver" nations will help prevent attacks like those on Sept. 11, 2001, and the administration is asking Congress to delay the deadline for biometric passports by two years because the 27 countries won't be able to meet the original deadline. Except for travelers from Mexico and Canada, all other visitors are already being fingerprinted and photographed.

"It will add security by allowing us to check against our terrorist criminal watch list those foreign visitors who are traveling" from visa waiver countries, said Homeland Security Department Undersecretary Asa Hutchinson.

But some counterterrorism experts are skeptical that the enhanced airport security measures will help nab terrorists before they strike. Vincent Cannistraro, former director of the CIA's counterterrorism center, said the fingerprint data will be useful only to identify a terrorist after he has struck.

Nevertheless, even Australia, which had 400,000 of its citizens visit the United States last year, said it would not object. "We were ready to go," said embassy spokesman Matt Francis. "But it's entirely a decision of U.S. authorities."

In related news, according to wire reports:

The Bush administration gave the federal panel reviewing the Sept. 11 attacks access Friday to thousands of classified counterterrorism documents from the Clinton administration.

Bush officials granted the commission's request to review the material after Bruce Lindsey, former legal adviser to President Clinton, said the commission isn't getting a full picture of Clinton's terrorism policies because the Bush administration had only forwarded 25 percent of the 11,000 records it wanted to provide the panel.

The European chief of an outlawed Marxist group from Turkey was arrested in Italy this week as 10 more suspected group members were picked up across Europe, investigators said Friday. The arrests raise to 63 the number of people arrested in Turkey, Italy, Belgium, Germany and the Netherlands in recent days on suspicion of having ties to the Marxist Revolutionary People's Liberation Army/Front, or DHKP-C.

Four jailed Malaysians admitted they were part of an Islamic terrorist group -- Jemaah Islamiyah -- and that a string of attacks against churches and other targets in Southeast Asia -- including bombings in Bali that killed 202 people -- was inspired by Osama bin Laden.

A string of bombings and clashes with police this week in Uzbekistan were the work of a group with foreign ties, the chief prosecutor of the Central Asian country said Friday.

Ten policemen, 33 suspected Islamic militants and four civilians were killed in the series of incidents that began Sunday night, Prosecutor General Rashid Kadyrov said. Kadyrov did not name any group or groups suspected in the attacks, but authorities have previously said they suspect the bombers have ties to al-Qaida.