WASHINGTON (AP) -- The question has never stopped being asked since the morning of Sept. 11, 2001: What next?

Billions of dollars and the efforts of untold thousands of people around the clock are aimed squarely at finding the answer.

The next big terrorist attack could be cooked up in a lab, giving new meaning to technology at the cutting - or deadly - edge. It could come again from the sky, in a frighteningly similar way. Or in some completely new way. Even now officials are chasing a "specific, credible but unconfirmed" threat of a car-bomb attack in New York or Washington, timed to the 9/11 anniversary period.

Everyone knows that plotters are dreaming of a new strike. The necessary presumption, even if it never happens, is that it surely will.

"It may be next week, it may be 10 years from now, we just don't know," Lee Hamilton, top Democrat on the commission that studied the 9/11 attacks, told The Associated Press. The expectation has to be that "sometime they will get through, the terrorists, and attack us again."

As the Sunday 10-year anniversary approaches, counterterrorism officials and law enforcement are shoring up security and publicly warning people about a potential "active plot" to launch a car-bomb attack in one of the two cities struck on 9/11. They are searching for three possible plotters who might have slipped into the country.

More broadly, the people devoted to identifying and averting the next attack are examining everything from the obvious to the fanciful. Nothing is too outlandish to be considered. The nation, after all, learned a decade ago that what seemed farfetched one day, came out of the blue the next.

Terrorists are known to be probing for weakness and opportunity on many broad fronts. A look at the major ones:

PLANES AND TRAINS

There's a reason you're taking off your shoes at airport checkpoints and walking through high-tech machines that show a naked image of your body.

Although terrorists have not successfully attacked commercial airliners since 9/11, they've still got their sights set on the skies.

Of all the intelligence about terrorism plots and tactics, officials most often see threats directed at planes. Al-Qaida especially seems preoccupied with using commercial aircraft as weapons, presumably since this was the method for its most successful attack ever against the West.

Intelligence officials are waiting for the next iteration of the printer bomb - a first-of-its-kind device built by a Yemeni who hid an industrial explosive in the cartridge of a printer. When X-rayed, the explosive looked like the cartridge's ink powder. Had it not been for good intelligence, those bombs could have blown up cargo planes over major U.S. cities in October 2010.

Rail systems continue to be attractive targets as they generally have less stringent security than airports. A successful subway attack has the potential to drastically change the day-to-day lives of Americans who use public transit 34 million times each weekday. London and Madrid have fallen victim to attacks since 9/11, and the U.S. has foiled plots targeting the rail systems in New York City and Washington.

Evidence found in Osama bin Laden's compound after he was killed showed that al-Qaida was looking at the structures of trains and train tracks, seeking the best spot for a derailment that would yield the most fatalities. That was just one indication the threat to planes and trains is not going away soon.

The latest possible threat identified bridges or tunnels as among the potential targets. In New York City, authorities were stopping vehicles at the 59th Street bridge Friday, while National Guard troops and transit police, carrying assault rifles, watched crowds at Penn Station. Police departments in New York and Washington deployed even more forces than had been planned for the anniversary period.

-AP Writer Eileen Sullivan

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BIOTERRORISM

A decade's advances in biology have put infectious and chemical agents more easily within the reach of terrorists - a prospect that was already scary back then.

Because of improvements in genome sequencing and the new field of synthetic biology, for example, scientists fear that smallpox, a once catastrophic disease now conquered, can be replicated by terrorists without having to acquire the only known remaining smallpox germs, held in tightly secure labs in the U.S. and Russia.

"We can create the agents that we know of from the world, like smallpox, from scratch," said Dr. Thomas Ingelsby, director of the Center for Biosecurity. Scientists did just that with the polio virus in 2003, and hundreds or thousands may be able to do it with smallpox or other bioterrorism agents now, said Amy Smithson, a senior fellow at the Center for Nonproliferation Studies.

The march of science, though, also helps to protect the population, says Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease. Better vaccines have been developed. The public health system is stronger. Most important, scientists can now detect and genomically sequence bioterrorism agents within hours instead of the weeks it used to take, Fauci said.

Although those improved defenses have not been tested against a bio-attack, they have shown their worth against natural disease such as pandemic flu and SARS, Fauci says. "Nature is the worst bioterrorist."

Just when the U.S. was dealing with the 9/11 aftermath, attacks using anthrax spores, sent through the mail in 2001, killed five people and sickened 17. Bioterrorism was the new scare. Nearly $67 billion would be spent on biosecurity over the decade, according to a study by Ingelsby's center.

Since the anthrax letters stopped - the FBI suspects the attacker was a federal scientist who committed suicide in 2008 - there have been no biological or chemical terrorist attacks in the U.S.

Experts aren't sure why. It could be luck, or better security. It could be that terrorists find this sort of attack too tough to pull off.

It remains debatable what terrorists are capable of doing with bioweapons, Smithson says. "Scaring the bejesus out of us? Oh, yes. Killing in modest numbers? Yes indeed." But mass death? She says that is still unknown.

-AP Science Writer Seth Borenstein

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MASS SHOOTINGS AND BOMBINGS

The commando-style assault on Mumbai in 2008 forced police departments in the U.S. and everywhere to rethink how they respond to the threat of roving gunmen.

Before Mumbai, police officials say, much of the focus for SWAT and other emergency response teams was to contain threats and be prepared to negotiate the release of hostages.

But Mumbai introduced a frightening dynamic: tightly coordinated, heavily armed terrorists attacking multiple urban targets simultaneously, hostages being taken only so they could be executed and shooters expecting to die themselves.

Part of the challenge was to avoid being caught flat-footed and out-gunned like Mumbai authorities. Of the 50 police officers on duty at a train station there when gunfire erupted, only half had guns. The lone security guard at a targeted Jewish center was unarmed and ran for his life.

Immediately afterward, the Los Angeles Police Department - which had two SWAT teams at the time - began making ordinary patrol officers carry assault weapons in the trunks of their cars for use in case of a mass attack, said William Bratton, a security firm executive who's headed both the LAPD and the New York Police Department.

"The issue became about firepower and personnel," Bratton said. "We had to refocus and retrain to deal with a new threat."

The NYPD, which has 400 Emergency Service Unit officers, has trained scores of additional officers on the use of high-powered rifles loaded with armor-piercing bullets. The officers have been put through drills at a facility that simulates a typical New York City block.

The use of motorboats by the Mumbai terrorists also prompted New York police to order a high-speed combat vessel that can carry up to 30 officers armed with heavy weapons. Officials say the $5 million boat gives the department the ability to respond with rapid force if the Statue of Liberty, Ellis Island or other sites are faced with a threat like this year's Norway massacre.

Although U.S. police forces are no strangers to gun violence, the attack in Norway has them thinking about how to prepare for a lone killer out to commit mass murder in places less accessible than a city street, suburban town house or country road.

-AP Writer Tom Hays

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CYBERTERRORISM

Because all sorts of people can sit down with computers and try to make mischief, it's safe to say the U.S. government is already routinely under cyberattack. Not much has come of it, as far as is known. Still, some breaches have been jarring.

Cybersecurity experts and government officials don't believe terrorist organizations have the technical skills yet to pull off a devastating attack on crucial U.S. computer networks. They do worry that such a capability might come.

Defense Secretary Leon Panetta has gone so far as to predict that the escalating threat could be the next Pearl Harbor - an attack triggering a war.

U.S. agencies, government contractors, financial institutions and other key businesses are being probed and attacked millions of times a day by terrorists, rogue nations, criminals and other hackers. Experts agree the U.S. is not prepared to counter, or often even detect, many intrusions.

Over the past two years, there have been notable breaches of the Pentagon, military contractors and the nation's electrical grid. And last year, officials raised alarms about hackers targeting power plants and other critical operations either to bring them down or take them over. Panetta told lawmakers a sophisticated attack could have a "strong likelihood" of crippling the nation's power, financial and government systems.

The U.S. expects cyberattacks to be part of any future war or major confrontation. And while U.S. military power is a strong deterrent against overtly destructive attacks, Pentagon officials say terrorist groups and rogue states are more likely to strike with little hesitation, and are a more unpredictable threat.

Indeed, Iranian officials have acknowledged, if not boasted, about their "cyber army" attacking websites of Tehran's perceived enemies. Iran has been a victim, too, as hackers penetrated the country's nuclear program with the Stuxnet computer worm and others. U.N. officials said the Stuxnet attack may have been responsible for a temporary shutdown of Iran's enrichment program.

-AP Writer Lolita C. Baldor

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NUCLEAR TERRORISM

Call it the low-probability, high-consequence threat.

Attainment of nuclear weaponry is a steep climb for a nation, much less a rogue organization. And with the U.S. deploying an array of radiation-detection sensors and aggressively monitoring source material, the odds may be slim of a terrorist group detonating a small bomb in the West. But the consequences could be so catastrophic that the U.S. takes the threat seriously. And officials believe it may be growing.

Many experts are increasingly worried about instability in nuclear-armed Pakistan.

Gary Samore, White House coordinator for arms control, told the Arms Control Association in May that "even the best nuclear security measures might break down" in Pakistan, which is under tremendous pressure from battle-hardened militant groups and in recent years has suffered brazen attacks on major government and military installations.

For the most part, U.S. officials have expressed confidence in Pakistan's armed forces and the steps being taken to protect the nation's 90 to 100 nuclear warheads.

If Pakistan were to implode and terrorists got their hands on a weapon, they would become the targets of the greatest manhunt the world has ever seen. Just holding onto such a prize would be a formidable challenge, Somehow getting it to the U.S. would be even harder.

But U.S. officials consider nuclear terrorism the single biggest threat to national security. That's because even if the chances of it happening are very low, the damage from such an attack would dwarf those of 9/11 and all other such strikes combined.

According to a 2003 Harvard report, a moderate-sized nuclear blast at Grand Central Station in Manhattan could kill half a million people and do $1 trillion in property damage.

Officials have long feared that al-Qaida planned to use "dirty bombs," conventional explosives that scatter radioactive materials but don't produce a nuclear blast. Experts say they would kill a relatively small number of people but could disrupt commerce and government, sow panic and require a massive cleanup.

The U.S. is working with other nations to lock up radiological materials to prevent their use by terrorists, but it's a daunting task: Those materials are widely used in research, medicine and industry all over the world.

According to a federal report, enough radiation monitors have been installed to scan nearly all cargo and vehicles entering the U.S. through land borders and major seaports. But the U.S. still does not scan all rail cars, international air cargo or commercial aviation aircraft, their passengers and baggage.

-AP Writer Doug Birch

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THE CRUCIAL UNKNOWN

It seems one of the best ways to figure out what terrorists might do next is to pretend you are one.

U.S. analysts study what terrorists have done in the past, what they've talked about on jihadist forums and what they're caught discussing in intercepts. Then the analysts consider how they would carry out an attack if they were in the terrorists' shoes. At the very least, this provides certain patterns to watch for.

Teams of officials are devoted to the tricky and crucial question of what new methods terrorists might use.

Much of the intelligence that helps to predict new methods of attack comes from old-fashioned spying: gleaning tips from human networks or listening in on communication spoken over cellphones or typed online.

Those human source networks have been reconstituted after budget cuts decimated spy networks in the 1990s. Computer sleuthing has been expanded in scope and authority post-9/11, enabling analysts to monitor and penetrate jihadist websites more aggressively to see what ideas are percolating and how much traffic such ideas generate.

The search for attack possibilities ranges far and wide. The CIA has even brought in Hollywood scriptwriters to come up with their most fantastic, yet doable, notions to cause mass destruction.

But most ideas come from "Red Team" analysts used by the National Counterterrorism Center and CIA to challenge the prevailing analysis and think as creatively as plotters do.

"We're all worried about aircraft, for instance," said Mike Leiter, just-retired head of the center. "So we get a Red Team together to say if you were a terror operator, what's an easier way to go after us?"

When the CIA suspected it had bin Laden in its sights in Pakistan, the intelligence community set up a Red Team of analysts who were not involved with the original project, to question every aspect of the bin Laden team's analysis and come up with logical reasons he might not be in the compound after all. In this case, the second team backed the original team's conclusions, which proved right.

- AP Intelligence Writer Kimberly Dozier

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Associated Press writer Nancy Benac contributed to this report.