The government Friday raised the terrorist threat index for only the second time, warning that newly acquired intelligence indicates a "high risk" of attacks by the al Qaeda terrorist network against U.S. targets at home and abroad.

U.S. intelligence and health officials said in public announcements and private briefings that they were particularly concerned about chemical, biological or radiological weapons, including ricin, cyanide and "dirty bombs" that would spread radioactive debris over a wide area.

Attorney General John D. Ashcroft and other officials said al Qaeda operatives may seek to time attacks to coincide with the hajj, the annual five-day pilgrimage by Muslims to Mecca, Saudi Arabia, that begins Sunday. Homeland Security Secretary Tom Ridge urged citizens to "to prepare for an emergency" by readying contact lists, talking with their families and reviewing information on other precautions.

The heightened alert status, from yellow to orange on the five-tiered, color-coded scale, is the result of days of contentious debate within U.S. counterterrorism circles over what to do about a surge in intelligence information indicating the possibility of attack. Many analysts, particularly in the White House and Pentagon, had pushed for a general public warning like the one issued Friday. But some FBI and CIA officials had argued that the information was too vague and could unnecessarily alarm the public.

Ultimately, President Bush made the final decision, raising the 11-month-old threat index to its second-highest level. The designation triggered tighter security at borders, airports and hotels, enhanced identification checks at government buildings and beefed-up protection of power grids, dams, financial networks and transportation systems. Officials also urged greater vigilance by all Americans.

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.

However, others with access to the intelligence upon which the alert was based said it was largely an effort to make sure government officials could not be blamed for not warning Americans, as they were after the Sept. 11, 2001, attacks. "That's what this whole process is about," said one well-placed intelligence source. They said the information was voluminous but not specific.

The warning follows a wave of high-profile terrorism arrests throughout Europe in recent weeks. Ashcroft said U.S. officials were particularly concerned about the arrests of more than a dozen men, mostly Algerians, in London after authorities there discovered traces of the deadly toxin ricin. The State Department on Thursday issued a "worldwide caution," warning of a growing threat of attacks by al Qaeda or other terrorist groups with chemical or biological weapons.

The FBI also announced separately on Friday that it was seeking a Pakistani man, Mohammed Sher Mohammad Khan, who is wanted for questioning and is believed to have entered the United States illegally sometime after September 2001. His case was "one of a number of factors taken into consideration" in elevating the threat index, a law enforcement official said.

Since it was unveiled last March, the terror threat warning system has generally been held at yellow, which signifies an elevated risk of attack. Yesterday's decision to raise the threat level to orange, indicating high risk, was only the second time the index has been moved up. The last time was on the eve of the anniversary of the Sept. 11, 2001, attacks. The only level above orange is red, which indicates an imminent or ongoing attack.

Ashcroft, joined at an afternoon news conference by Ridge and FBI Director Robert S. Mueller III, said the decision to elevate the threat index was based on "specific intelligence" that had been "corroborated by multiple intelligence services."

Ashcroft warned about the threat of attacks on "soft or lightly guarded targets" in the United States, including apartment buildings and hotels. Ridge took the unusual step of urging citizens to take "precautionary measures" to "minimize the damage" from a possible terrorist attack, and to consider a "contact plan" in the event of an emergency. Yet Ridge and other officials also encouraged people not to drastically change their plans or lifestyles.

Homeland Security officials held urgent briefings throughout the day with police chiefs, sheriffs, public health directors and other community leaders from around the country. Officials said that security would be noticeably tightened in public locations, while health authorities issued alerts urging that hospitals and physicians review procedures for treating victims of radiation and chemical exposures.

Staff writers Ceci Connolly, Spencer S. Hsu, Dana Priest and Susan Schmidt contributed to this report.