A specific, credible but unconfirmed terrorist threat to residents of New York City and Washington, D.C., was brought to the public's attention Thursday evening, just three days before the 10th anniversary of the September 11 attacks on those two cities. In the past decade such alerts from government and public safety officials have been all too common as home-grown and international terrorists alike have attempted to use a variety of methods to inflict widespread damage on the U.S. and its residents. So the question remaining these days is: What's likely to be next among all the possible threats?

The CIA notes the annual U.S. death rate is 8.38 fatalities per 1,000 citizens, below that of a country like Nigeria but above other places, such as Uzbekistan. The leading causes of death in the U.S. are heart disease, cancer and car accidents, which killed roughly 1.2 million Americans in 2007, according to the U.S. Centers for Disease Control—more than half of all fatalities in the country. For comparison, terrorists killed no one in the U.S. that year.

But, judging by previous assaults and interviews with experts, the following covers a short-list of real and perceived terrorist threats in order of likelihood, from most to least.

On December 22, 2001, Richard Reid attempted to light his shoe on fire on American Airlines Flight 63. His shoe sole contained a chemical explosive—and ignited a security regulation for removing shoes at U.S. airport security checkpoints that persists to this day.

But Reid is hardly the first to attempt to use chemical explosives to terrorize the population. Favorite compounds for such efforts include pentaerythritol tetranitrate (PETN), triacetone triperoxide (TATP) and nitroglycerin, among others. The ingredients for some such explosives can be found at the local pharmacy and are hard to identify individually by current airport security technology, hence the prohibition against bringing anything but very small quantities of liquids on board an aircraft.

In the 1990s terrorists succeeded in smuggling liquid chemical explosives onto an aircraft from the Philippines in the type of saline solution bottle typically carried by contact lens wearers, detonated it and killed a fellow passenger. Subsequently, terrorists have tried everything from sewing bombs into underwear to smuggling explosives onto flights in packages bearing printer cartridges.

Such incendiaries have been used time and time again to attack transportation, from car bombs, like the one that fizzled in New York City's Times Square in May 2010, to bombing planes. In 2006 terrorists plotted to blow up 10 planes simultaneously, and failed thanks to covert intelligence efforts by the U.K., U.S. and other countries. In Russia, terrorists did detonate bombs on board two planes in 2004, killing all 89 passengers and crew. In 1988 a bomb destroyed Pan Am Flight 103, which fell from the sky above Lockerbie, Scotland, taking 259 lives on the plane as well as 11 on the ground.

Urban mass transit also attracts terrorist bomb efforts, such as the suicide bombings on London trains and buses on July 7, 2005. The files seized after Osama bin Laden's assassination revealed plots to attack U.S. railways and detonate car bombs in similar fashion.

Safety monitoring systems ensure that a nuclear power plant doesn't begin to melt down by detecting temperature increases in reactor coolant water and fuel rods, along with other anomalous events in time to take preventive action. But on January 25, 2003, such systems shut down for nearly five hours at the Davis–Besse Nuclear Power Station in Oak Harbor, Ohio. Why? The "SQL Slammer" worm had infected the computers that run those systems via a contractor's computer. Fortunately, the plant had been shut down since 2002 to deal with another safety malfunction—a hole and cracks in its reactor.

This risk is not confined to power plants, nuclear or otherwise. The entire electrical grid is vulnerable to hacking, as proved by a test attack—dubbed Aurora—carried out by the Idaho National Laboratory. Nor is such hacking confined to the energy infrastructure. The U.S. Department of Defense, Google, bank ATMs and even the very microchips that make computing possible have all been hacked to nefarious ends.

On September 11, 2001, terrorists used airplanes themselves as weapons to bring down the Twin Towers of the World Trade Center in New York City as well as inflict severe damage on the Pentagon in Washington, D.C. Airplanes continue to hold a peculiar fascination for terrorists, although the bulk of such plots focus on detonating chemical explosives rather than commandeering and using the jets themselves.

As a result, airplanes remain the focus of many people's fears. But statisticians have calculated that the odds of being involved in a terrorist-related airplane death stand at more than one in 10,000,000, based on the past decade of air travel. For comparison, the odds of being struck by lightning are one in 500,000.

On March 20, 1995, members of a religious cult released poisonous sarin gas on five trains in the Tokyo subway system. The colorless, odorless nerve agent immediately caused sweating and muscle twitching, among other symptoms, some of it severe enough to kill 12 people by arresting their breathing. The cult, it was later discovered, also had in its arsenal hydrogen cyanide gas and other chemical weapons capable of inflicting even more massive casualties.

The idea of using nerve gas, poisons or other chemical weapons is not restricted to crazed Japanese cults. U.S. security officials have implicated the terrorist network al Qaeda in attempts to secure large quantities of castor beans. The beans are the primary ingredient in ricin, a white powder, which is deadly if inhaled or otherwise absorbed in minute quantities.

Beginning on September 18, 2001, envelopes arrived at the offices of Democratic Sens. Tom Daschle (S.D.) and Patrick Leahy (Vt.) in Washington, D.C., as well as media organizations such as ABC, CBS, NBC in New York City and American Media in Boca Raton, Fla. Each bore a powder—the spores of a bacterium that can cause a deadly infection known as anthrax. In fact, five people died from subsequent illnesses caused by the anthrax exposure.

Anthrax is hardly the only biological weapon, which range from directly infectious bacteria to toxin-producing microbes. The U.S. and Russia both had research programs to weaponize organisms in previous decades[OR: during the Cold War], as did other countries. Although banned by international treaty, some nations continue to develop and/or stockpile them, according to the Arms Control Association. But complicated technology is not required to wreak this type of havoc; cult leaders in Oregon infected local salad bars with diarrhea-causing salmonella in a bid to influence an election in 1984.

The U.S. has spent the past several decades turning old Russian nuclear warheads into nuclear fuel for its own reactors—along with installing more security at Russia's remaining nuclear weapon sites—thereby diminishing the threat of nuclear apocalypse. But some critics, such as journalist William Langeswiesche, have noted how the remaining Russian arsenal remains vulnerable to theft or sale. The idea may not be to launch a full-fledged nuclear bomb assault but rather to create a so-called dirty bomb—a conventional chemical explosive packed with radioactive material that can be detonated in any number of ways and sizes.

Such a bomb would likely directly kill only those caught in its initial explosion but—as the recent meltdowns at the Fukushima nuclear power plant in Japan show—fear of radioactive contamination might spur panic.