Updated

There is not enough specific, credible evidence to believe another attack on America is imminent, despite warnings of increased terrorist activity, U.S. officials said Thursday.

The nation's terrorist threat level remains at code yellow, or "elevated," the middle of a five-point scale of risk developed by the Bush administration after the Sept 11, 2001, attacks.

Yet there is clearly worry that the danger is growing because of increased "chatter" picked up through intelligence channels, the continuing U.S. showdown with Iraq and the recently revealed taped warnings believed to be from Al Qaeda leader Usama bin Laden.

"While we are not raising the threat level, we are taking additional precautions to meet the threat," White House spokesman Scott McClellan said.

These include unspecified "additional steps to ramp up our protection and prevention measures" within federal agencies, McClellan said. The FBI and other agencies also are communicating possible threats and assessments of risk to state and local law enforcement agencies and specific industries that could be targeted.

In recent weeks, the FBI has issued warnings about possible attacks on U.S. railroads and on the energy industry, as well as a more general warning about heightened risk during the Muslim holy month of Ramadan, which started Wednesday and ends Dec. 5.

"We're especially sensitive to timeframes which might be thought by the enemy to be a time when they might want to make a statement," Attorney General John Ashcroft said.

On Wednesday, the FBI told authorities in Houston, Chicago, San Francisco and Washington to be aware of threats against hospitals. Even though that threat was assigned low credibility by senior law enforcement officials, the FBI is preferring to err on the side of caution in terms of giving out information, officials said.

The idea is to increase vigilance among local police and people working in industries that are potential targets.

The recent nightclub bombing in Bali, Indonesia, the assault on Marines in Kuwait, and the attack on a French oil tanker near Yemen — as well as the U.S. strike on a car carrying suspected terrorists, also in Yemen — are described by several law enforcement officials as actions that point to an increased threat.

"If there was any doubt in anybody's mind that Al Qaeda remains a dangerous threat to America or the world, I suspect it was dispelled with the string of attacks," Tom Ridge, director of the White House homeland security office, said Thursday.

It is up to Ridge and Ashcroft to decide whether a change in threat level is warranted. Ashcroft and Deputy Attorney General Larry Thompson are among a few Justice Department officials who see the daily raw intelligence on terrorism gathered by the FBI, CIA and other intelligence agencies.

The threat level was elevated from yellow to orange for two weeks in September, to coincide with the first anniversary of the Sept. 11 attacks. It has remained at yellow since then, but the tense U.S.-Iraq showdown has lawmakers and the Bush administration on guard.

"I think that as we ratchet up toward Iraq, we have to believe that there will be attempts in this country anywhere, perhaps everywhere, to do us harm," Sen. Richard Shelby of Alabama, senior Republican on the Senate Intelligence Committee, said in a televised interview.

Ridge and FBI Director Robert Mueller say the nation is far better prepared to detect and stop a terrorist attack than it was prior to Sept. 11, 2001. They say the intelligence sharing among agencies is vastly improved, as well as information about airplane passengers, people who enter through U.S. border crossings and students who lose their status and remain in this country.

Still, it remains a monumental task for analysts to sort through the information.

"One of the most difficult tasks in the intelligence community is segregating the information that may be helpful and useful from the disinformation that's looking to take us down the wrong path," Ridge said.