Counterterrorism and law enforcement officials told Fox News Tuesday that they are extremely concerned that terrorist groups like Al Qaeda (search) may be planning an attack during one of the major events scheduled for this summer.

The comments came after a think tank study revealed that despite the elimination of several key figures, Al Qaeda still has a functioning leadership, over 18,000 potential terrorists in its global network and a swelling membership thanks to the war in Iraq.

"There is significant concern," one counterterrorism official told Fox News.

The intelligence was described Tuesday by another senior counterterrorism official on condition of anonymity as extremely credible and backed by an unusually high level of corroboration.

The intelligence does not include a time, place or method of attack but is among the most disturbing received by the government since the Al Qaeda attacks of Sept. 11, 2001, the official said.

"There is clearly a steady drumbeat of information that they are going to attack and hit us hard," said another senior federal counterterrorism official, who described the intelligence as highly credible.

However, officials said the threat level would not be immediately be raised from yellow, or elevated, to orange, or high. The threat level has been at yellow — midpoint on the five-color scale — since January.

Los Angeles police held a news conference Tuesday to reassure the public. "We would be foolhardy to ignore those statements, but I think it would be irresponsible to panic," said John Miller, head of the LAPD counterterrorism bureau.

New York Police Commissioner Raymond Kelly said city officials have not been advised that terrorists are plotting an attack there.

"We are receiving highly sensitive intelligence information on a regular basis, including today, and there is nothing in that reporting to indicate a specific threat or looming attack against New York City," Kelly said in a statement.

Attorney General John Ashcroft (search) and FBI Director Robert Mueller (search) plan a news conference Wednesday to outline an intensive effort by law enforcement, intelligence and homeland security officials to detect and disrupt any potential plots. And the FBI plans to dispatch a bulletin to some 18,000 state and local law enforcement agencies warning of the threat.

The FBI also has already created a special task force that is focused solely on dealing with this summer's threat. The task force, whose existence until recently was classified, is intended to ensure that no valuable bits of information or intelligence fall through the cracks — as happened repeatedly before the Sept. 11 attacks.

Other actions to be taken include new FBI interviews with people who may have provided valuable information in the past and a fresh examination of older investigative leads to determine if they might point to elements of the summer plot.

Officials said they are concerned about some "high value targets," including the dedication of the World War II memorial planned for May 29 in Washington, D.C., the Olympics and the presidential conventions.

Counterterrorism officials said the concern is based on recent "source work" including intercepted communications, and the FBI will be working with law enforcement partners and the Joint Terrorism Task Forces to assess information that is coming in.

"We will be reviewing every lead we've had for the past year to see if taken in today's light it means something different," one counterterrorism official said.

Officials did not say what kind of attack to expect. However, Retired Lt. Gen. Patrick Hughes (search), the top intelligence official at the Homeland Security Department, warned earlier this month Al Qaeda may be planning to strike the United States with a chemical or biological weapon.

Anthrax is easy to produce and disperse, Hughes said, noting that the recipes for it and the deadly poison, ricin, are on the Internet.

"If the past is indeed prologue, then we are going to screw up, or they are going to get lucky," Hughes said. "I can't sleep."

'Al Qaeda Has Fully Reconstituted'

The study, released by the International Institute of Strategic Studies (search ), says Al Qaeda poses a growing threat to Western interests and attacks are likely to increase.

The Strategic Survey 2003-4 says the United States remains the prime target of the group, which is operating in an estimated 60-plus nations around the world. An Al Qaeda leader has said 4 million Americans will have to be killed "as a prerequisite to any Islamic victory."

"Al Qaeda must be expected to keep trying to develop more promising plans for terrorist operations in North America and Europe, potentially involving weapons of mass destruction," institute director John Chipman (search) told a news conference to launch the annual survey.

At the same time, it will continue carrying out attacks on "soft targets encompassing Americans, Europeans and Israelis and aiding the insurgency in Iraq," he added.

Monday the Search for International Terrorist Entities Institute (search) released translated portions of Al Qaeda's Al-Battar manual's kidnapping instructions on its Web site.

SITE Institute lists in detail the instructions operatives are given for different types of kidnappings, the different stages to go through during an abduction and how to deal with a hostage in a kidnapping situation.

The Strategic Survey's estimate of 18,000 Al Qaeda fighters was based on intelligence estimates that the organization trained at least 20,000 fighters in its training camps in Afghanistan before the United States and its allies ousted the Taliban regime.

In the ensuing war on terror, some 2,000 Al Qaeda fighters have been killed or captured, the survey said.

Iraq has become the new magnet of Al Qaeda's war against the United States and up to 1,000 foreign Islamic fighters have infiltrated Iraqi territory, where they are cooperating with Iraqi forces, the survey said.

Al Qaeda appears to have successfully reconstituted its operations in dispersed groups and through local allies since being driven out of Afghanistan, the survey said.

"The Madrid bombings in March 2004 suggested that Al Qaeda had fully reconstituted, set its sights firmly on the U.S. and its closest Western allies in Europe, and established a new and effective modus operandi," the survey said.

The U.S.-led war in Iraq has increased the risk to Western interests in Arab countries, the survey said.

The West and its allies must continue to mount a major offensive against Al Qaeda and progress will be incremental, the report said.

Any security offensive against Al Qaeda must be accompanied with political developments, such as the democratization of Iraq and the resolution of conflict in Israel, it said.

Progress against Al Qaeda "is likely to accelerate only with currently elusive political developments that would broadly depress recruitment and motivation," the report said.

Summer Packed With High-Profile Events

Beginning with Saturday's dedication of the new World War II Memorial in Washington, the summer presents a number of high-profile targets in the United States. They include the G-8 summit in Georgia next month that will attract top officials from some of America's closest allies, the Democratic National Convention in Boston in July and the Republican National Convention in August in New York.

The FBI and Homeland Security Department also are concerned about so-called soft targets such as shopping malls anywhere in the United States that offer a far less protected environment than a political convention hall.

U.S. authorities repeatedly have said Al Qaeda is determined to mount an attack on U.S. soil, in part to announce to the world that it remains capable of doing so despite the money and effort that has gone into homeland security in the wake of the Sept. 11 attacks.

There also is concern terrorists might try to mount an attack to coincide with the November election. The political fallout from the March 11 train bombings (search) in Spain taught Al Qaeda that an attack timed to an election can have a major impact. Spain's former ruling party was ousted in the voting that followed the bombing, which killed 191 and injured more than 2,000.

The official did not say how many suspected Al Qaeda or other terrorist operatives are believed in the country, whether they made their way into the United States recently or have been here for some time. The FBI has warned in the past that Islamic extremist groups may attempt to recruit non-Middle Easterners or women for attacks because they would be less likely to arouse suspicion.

Special security attention already is being focused to the nation's rail, subway and bus lines. The FBI last week sent out an intelligence bulletin to law enforcement agencies urging vigilance against suicide bombers, who have been used by terror groups worldwide to devastating effect but not so far in the United States.

Separately, Immigration and Customs Enforcement chief Michael Garcia told reporters Tuesday that some 2,300 of its agents are being deployed to assist in security for the high-profile events scheduled this summer in the United States. These include as many as 20 agents each day working with the Secret Service to protect the campaigns of President Bush and Sen. John Kerry, the Democratic presidential candidate.

Garcia said his agency also is working to "tighten the investigative system" to ensure that terrorists do not enter the United States by way of human smuggling operations or through the vast, largely unprotected border with Canada.

Fox News' Anna Stolley and The Associated Press contributed to this report.