Washington ? U.S. officials elevated the national terrorism alert to the second highest level Friday and warned of possible al-Qaida attacks on a sweeping range of potential targets from apartment houses to “symbols of American power.”

As attention shifted from the Persian Gulf to the home front, Bush administration officials said a torrent of terrorist activity around the world suggested that surviving cells of Osama bin Laden’s network could attack Americans by the end of the next week.

They said intelligence agencies had intercepted more al-Qaida e-mails, phone calls and other communications than at any time since just before the attacks of Sept. 11, 2001. Much of the information has been corroborated by secret agents, they said.

Intelligence officials said two independent “armies” of al-Qaida operatives, an estimated 31 people in all, could be at large in the United States and Canada, plotting to exploit the Iraqi crisis through a “creative and imaginative” attack.

The FBI asked the public for help Friday in locating a Pakistani man identified as Mohammed Sher Mohammed Khan, 36, who it said may have entered the United States illegally after Sept. 1, 2001.

The agency said it had no specific information that Khan was a terrorist, though it wanted to question him. Officials said Khan wasn’t the sole reason for the heightened alert, but was one of the primary factors.

“He’s a principal,” one intelligence official said, speaking on condition of anonymity.

Another intelligence official, who also asked not to be identified, said officials feared that an attack could involve a chemical or biological weapon or a so-called “dirty” bomb, a crude radiological device.

In announcing the heightened state of alert, Atty. Gen. John Ashcroft publicly raised the specter of chemical, biological or radiological assaults, noting that alleged al-Qaida members were arrested recently in London, where the poison ricin was seized.

One intelligence official said the threat level was raised after officials received indications of a coming attack in a specific location by a specific terrorist cell. That official wouldn’t elaborate, but another suggested that New York was the possible site.

Sen. Pat Roberts, R-Kan., chairman of the Senate intelligence committee, said the threat information “is serious and specific. … There’s not just chatter, but a pattern, and not just a pattern, but dots” that intelligence agencies have been able to connect in a general way.

Khan

“There are a lot of things that add up,” Roberts said.

New York Gov. George Pataki and New York City Mayor Michael Bloomberg told a news conference that doctors and nurses have been alerted to watch for unusual outbreaks of infectious disease. They also said they would activate special units of the state police and National Guard to help protect bridges, tunnels, subways and other facilities.

The timing of this latest warning, which covered Americans at home and abroad, coincided with the Muslim holy period of Hajj. That event begins Sunday, ends Thursday and typically produces enormous numbers of pilgrims from around the world to the holy city of Mecca in Saudi Arabia, straining the ability of security agencies to track known and suspected militants.

Targets plentiful

Ashcroft said bin Laden and other al-Qaida leaders “have emphasized planning for attacks on apartment buildings, hotels and other soft or lightly secured targets in the United States.”

Also at risk, he said, were economic targets and other “symbols of American power” and prestige. That was thought to be a reference to stock exchanges, the U.S. Capitol, the White House and similar institutions.

Authorities urged Americans to maintain their normal work and travel plans but enhance their general level of vigilance, an action they said could help deter attacks.

On the wider scale, they said more sky marshals would be assigned to commercial flights, security checks at airports would be intensified and travelers would be scrutinized more carefully at border crossings and other entry points.

“An alert public is our strongest asset,” said FBI Director Robert Mueller.

Mueller and other federal officials said the sheer magnitude of intercepted “chatter” — or terrorist communications — compelled them to elevate the alert from moderate to “high,” orange on the color-coded scale, just one step below the most ominous “severe,” or red alert.

The level was last raised to orange in September for the first anniversary of the attacks that killed thousands at the World Trade Center in New York, in the Pentagon near Washington and in a plane crash in Pennsylvania.

“This decision is based on specific intelligence received and analyzed by the full intelligence community,” Ashcroft said. “This information has been corroborated by multiple intelligence sources.”

Al-Qaida top worry

With the United States moving toward war with Iraq, authorities said they were worried about Iraqi agents but that their principal concern remained al-Qaida, which struck the United States with such savagery nearly 17 months ago.

“Since Sept. 11, the U.S. intelligence community has indicated that the al-Qaida terrorist network is still determined to attack innocent Americans, both here and abroad,” Ashcroft said.

‘Get informed’

As in the past, federal officials provided little practical advice for Americans.

They said families should prepare response plans, including places to meet in the wake of an attack or other emergency. They advised people seeking more information to visit www.dhs.gov on the Internet, the Department of Homeland Security’s Web site.

“All I would say to you, as a parent and a spouse, is: Take the time now to get informed,” said Homeland Security Secretary Tom Ridge.

Ridge and Ashcroft went to the Oval Office at 10:12 a.m. Friday and recommended that the president increase the level of alert, according to White House spokesman Ari Fleischer.

“I agree,” Bush replied. “Change the code.”

Administration and intelligence officials pointed to several threads of information that triggered Friday’s action.

First was a longstanding concern, based on debriefings of a high-ranking al-Qaida detainee, that bin Laden long ago dispatched three separate “armies” of terrorists, a total of about 50 people, to the United States and Canada.

One “army,” with 19 people, carried out the Sept. 11 suicide attacks. The two others remain at large and may not know of each other’s existence, according to officials who requested anonymity.

Another intelligence report indicated that bin Laden has recorded a new audio or video message, which analysts think is intended for broadcast soon after a major terrorist attack or series of attacks.

Bin Laden’s previous messages have criticized what he called U.S. aggression against the Iraqi people. He has lambasted U.N. sanctions and threats of war against Iraqi leader Saddam Hussein.