First developed as a weapon by the U.S. Army, VX is an oily, odorless and tasteless liquid that kills on contact with the skin or when inhaled in aerosol form. Like other nerve agents, it is treatable in the first minutes after exposure but otherwise leads swiftly to fatal convulsions and respiratory failure.

The United States, a signatory to the Chemical Weapons Convention, destroyed the last of its stocks of VX and other chemical agents on the Johnston Atoll, 825 miles southwest of Hawaii, in November 2000.

The U.N. Security Council ordered Iraq in April 1991 to relinquish all capabilities to make biological, chemical and nuclear weapons as well as long-range missiles.

During the morning rush hour on March 20, 1995, the Japanese cult Aum Shinrikyo placed packages on five subway trains converging on Tokyo's central station. When punctured, the packages spread vaporized Sarin through the subway cars and then into the stations as the trains pulled in.

Smallpox, which spreads by respiration and kills roughly one in three of those infected, took hundreds of millions of lives during a recorded history dating to Pharaonic Egypt. The last case was in 1978, and the disease was declared eradicated on May 8, 1980.

The U.S. government has known since the early 1990s about Soviet-era smallpox weapons, and collected circumstantial evidence of programs elsewhere.

The CIA now assesses that four nations - Iraq, North Korea, Russia and, to the surprise of some specialists, France - have undeclared samples of the smallpox virus.

For political and bureaucratic reasons, governments at all levels are telling far less to the public than to insiders about how to prepare for and behave in the initial chaos of a mass-casualty event.

Nearly all government advice on terrorism sacrifices practical particulars for an unalarming tone. The usual guidance is to maintain a three-day supply of food and water along with a radio, flashlight, batteries and first-aid kit.

Unsettling signs of al Qaeda's aims and skills in cyberspace have led some government experts to conclude that terrorists are at the threshold of using the Internet as a direct instrument of bloodshed.

Counterterrorism analysts have known for years that al Qaeda prepares for attacks with elaborate 'targeting packages' of photographs and notes.

Experts said public companies worry about the loss of customer confidence and the legal liability to shareholders or security vendors when they report flaws.

Ordinary Geiger counters, worn on belt clips and resembling pagers, have been in use by the U.S. Customs Service for years.

Searches of al Qaeda sites in Afghanistan, undertaken since American-backed forces took control there, are not known to have turned up a significant cache of nuclear materials.

There is evidence that some of al Qaeda's nuclear efforts over the years met with swindles and false leads.

Al Qaeda is closely aligned with the Chechens.

White House officials acknowledge in broad terms that a president's time and public rhetoric are among his most valuable policy tools.

The government of Sudan, employing a back channel direct from its president to the Central Intelligence Agency, offered in the early spring of 1996 to arrest Osama bin Laden and place him in Saudi custody, according to officials and former officials in all three countries.

Sudan expelled bin Laden on May 18, 1996, to Afghanistan.