Attorney General John D. Ashcroft and FBI Director Robert S. Mueller III warned today that al Qaeda is preparing to mount a large-scale terrorist attack "to hit the United States hard" this summer and asked Americans for help finding six men and one woman with terrorist ties.

Ashcroft said at a joint news conference that "credible intelligence from multiple sources indicate that al Qaeda plans to attempt an attack on the United States in the next few months."

Among the prominent events that terrorists may want to target, Ashcroft and Mueller said, are next month's Group of Eight summit in Sea Island, Ga., July 4 celebrations across the country and the Democratic and Republican national party conventions this summer in Boston and New York.

Citing the deadly bombings of commuter trains in Madrid in March, which came shortly before national elections that toppled the Spanish government, Ashcroft said al Qaeda appears to hope attacks "this summer or fall would lead to similar consequences" in the United States.

"Unfortunately, the same events that fill most of us with hope and pride are seen by terrorists as possible opportunities for attack," Mueller said.

Ashcroft and Mueller asked for the public's help in locating seven people who are suspected terrorist sympathizers. "They should all be considered armed and dangerous," Ashcroft said. They included:

* Aafia Siddiqui, 32, a Pakistani woman who has a doctorate in neurological science and has studied at the Massachusetts Institute of Technology and Brandeis University in the Boston area, as well as in Houston. Mueller said she may have left the Boston area in January 2003. An FBI fact sheet said she is believed to be in Pakistan.

* Adnan G. El Shukrijumah, 28, a suspected al Qaeda member who has been linked to Siddiqui and whose name has come up in interrogations of captured al Qaeda lieutenant Khalid Sheik Mohammed. A English-speaking pilot, Shukrijumah was born in Saudi Arabia and spent time in Florida. Ashcroft said Shukrijumah has attempted to reenter the United States using a variety of passports. "We know that he has been involved in terrorist planning with senior al Qaeda leaders overseas and has scouted sites across America that might be vulnerable to terrorist attack," Ashcroft added.

* Abderraouf Jdey, 38, a Tunisian-born Canadian citizen who has been sought since January 2002 and whose name, Mueller said, first surfaced on an al Qaeda video found in Afghanistan.

* Amer El-Maati, 41, a Kuwait-born pilot who Mueller said "is believed to have discussed hijacking a plane in Canada and flying it into a building in the United States."

* Adam Gadahn, 25, who Mueller said was a U.S. citizen from the West Coast who converted to Islam "in his youth" and attended terrorism training camps in Afghanistan. Mueller said Gadahn has served as a translator for al Qaeda.

* Fazul Abdullah Mohammed and Ahmed Khalfan Ghailani, who Mueller said were previously indicted in the United States and played roles in the 1998 bombings of two U.S. embassies in East Africa. Mohammed is an alleged al Qaeda leader in East Africa who also was a primary suspect in the November 2002 attack on an Israeli-owned hotel in Mombasa.

"While we do not have any reason at this time to believe that they are working in concert, we will not take any chances," Mueller said if the seven individuals sought.

Ashcroft noted that many of the seven spoke English and had experience living in the United States. He also said some might present themselves as Europeans and travel with families to avoid calling attention to themselves.

Ashcroft said the FBI has established a task force to coordinate intelligence and operations specifically related to the threat of imminent attacks. He and Mueller also encouraged Americans to report suspicious activities to authorities.

Federal officials said yesterday that the increased concerns about a possible attack on the United States were driven by intelligence deemed credible that was obtained about a month ago indicating an attack may be planned between now and Labor Day. That information dovetailed with other intelligence "chatter" suggesting that al Qaeda operatives are pleased with the change in government resulting from the March 11 terrorist bombings in Spain and may want to affect elections in the United States and other countries.

"They saw that an attack of that nature can have economic and political consequences and have some impact on the electoral process," said one federal official with access to counterterrorism intelligence.

Mueller said that law enforcement and security officials were taking "extraordinary precautions" to protect this summer's presidential nominating conventions. Intelligence and law enforcement officials also have been trying to strengthen security at the Summer Olympic Games in August in Athens and the World War II Memorial ceremony Saturday in the District.

Federal officials have been discussing raising the national threat level between now and Jan. 21, the day after the presidential inauguration, although Homeland Security Department officials have said no such announcement is scheduled.

"I can confirm that we have seen for the past several weeks a continuous stream of reporting that talks about the possibility of attacks on the United States," Homeland Security Secretary Tom Ridge said today during a round of television appearances on morning news shows.

Ridge said that intelligence, paired "with the number of high-profile events we'll have between now and the end of the year," has prompted a multi-agency effort "to see what we can do to accelerate the work that we do every single day to make us safer and more secure. And that's exactly what we're doing."

Ridge added: "We don't need to raise the threat level to bring together some of the best and brightest minds in this country to use people and technology to be safer."

Concerns that holiday travel and large public gatherings would be targeted by terrorists have prompted the Department of Homeland Security to elevate the national alert status in the past. Last May, Ridge raised the alert level to orange, or "high risk," for 10 days because of a "heightened vulnerability associated with the Memorial Day holiday." That move came after suicide attacks that killed 34 people in Riyadh, Saudi Arabia, and 43 in Casablanca, Morocco, earlier in the month raised fears that the al Qaeda terror network had regained its ability to mount international attacks.

Late last year, just before the Christmas and New Year's holidays, the alert level was raised again as U.S. officials canceled dozens of flights to the United States, including British Airways flights bound for Dulles International Airport, because of security concerns.

That was the fifth time since the system was put in place in March 2002 that Homeland Security officials officially changed the national alert status to orange. The alert level was officially lowered again -- to yellow, or "elevated," its current status -- on Jan. 9. But eight airports, including Washington's three major hubs, maintained a higher alert status until late March.

U.S. officials indicated that the new terrorism concerns were at least as serious as those that prompted raising the threat level last summer and before the December holidays.

"I'm more worried than I was at Christmastime," said one senior U.S. intelligence official, comparing the "election threat" to the canceling of specific airline flights around the holidays. He said the U.S. government is convinced there are as yet unidentified al Qaeda operatives residing in the United States, waiting for the word to launch plots.

"They are here, and there are indications they are preparing" attacks, said the official, whom government policy bars from being named.

Within the past three weeks, members of the House and Senate intelligence committees have received briefings from the CIA and the FBI on what the CIA counterterrorism center has termed the "election threat." The members have asked the agencies for more specific, follow-up briefings, including an assessment of al Qaeda's presence in the United States, congressional sources said.

One counterterrorism official said al Qaeda still aims to carry off an attack that would kill large numbers of people, and is aiming at modes of transportation such as airlines and ships. Anything less than a spectacular attack, such as a suicide bombing, would appear weak to al Qaeda's financiers, according to the counterterrorism official.

Mueller said today the exact nature of the threat of attack in the next several months was unclear. "Unfortunately, we currently do not know what form that threat may take," he said.

In April, an FBI bulletin to law enforcement agencies warned of possible truck bombs. A source familiar with the government's threat discussions said yesterday that truck bombs are a primary concern.

Another FBI bulletin, issued last week, urged law enforcement officials to be on the alert for possible suicide bombers. Officials were urged to take note of people dressed in bulky jackets in warm weather, clothing smelling of chemicals or trailing electrical wires, and they warned that potential bombers may be dressed in uniform or even disguised as pregnant women.

U.S. officials told The Washington Post in January that a major concern during the December alerts was that terrorists might employ a radiological weapon -- such as a "dirty bomb" that would use a conventional explosive to spew radioactive material and radiation across a small area. To search for such weapons, U.S. authorities deployed teams of casually dressed nuclear scientists with sophisticated radiation detection equipment hidden in briefcases and golf bags to Washington, New York, Las Vegas, Los Angeles, according to officials involved in the emergency effort. U.S. Coast Guard vessels also carried out stepped-up patrols of waterways in Baltimore and some other cities using radiation detectors.

President Bush has said two-thirds of al Qaeda's pre-Sept. 11, 2001, leadership has been killed or captured. But CIA Director George J. Tenet has said many established terrorist groups that previously did not work together are making a concerted effort to undermine the United States.