A sudden collapse of the government of Syrian President Bashar al-Assad could mean a breakdown in controls over the country’s weapons, U.S. officials and weapons experts said in interviews. But while Libya’s chemical arsenal consists of unwieldy canisters filled mostly with mustard gas, the World War I-era blistering agent, Syria possesses some of the deadliest chemicals ever to be weaponized, dispersed in thousands of artillery shells and warheads that are easy to transport.

Syria’s preferred poison is not mustard gas but sarin, the nerve agent that killed 13 people and sickened about 1,000 during a terrorist attack on the Tokyo subway system in 1995. Sarin, which is lethal if inhaled even in minute quantities, can also be used to contaminate water and food supplies.

Although many analysts doubt that Assad would deliberately share chemical bombs with terrorists, it is not inconceivable that weapons could vanish amid the chaos of an uprising that destroys Syria’s vaunted security services, which safeguard the munitions.

“This is a scenario that’s on the radar screen if things go downhill,” said a U.S. security official who monitors events in Syria. “A lot of people are watching this closely.”

Syria first developed chemical weapons in the 1970s and slowly amassed a sophisticated arsenal under the close supervision of then-President Hafez al-Assad and, later, his son Bashar, the current president. Using technology obtained in part from Russian scientists, the Assads sought to create a strategic deterrent against Israel, its vastly more powerful southern neighbor, whose forces humiliated Syrian troops in the 1967 Arab-Israeli war and captured the strategic Syrian plateau known as the Golan Heights.

Many countries, including the United States and Russia, gradually eliminated their chemical-weapons arsenals, but Syria refused to sign the U.N. Chemical Weapons convention and proceeded to develop an ever larger and deadlier stockpile. The CIA has concluded that Syria possesses a large stockpile of sarin-based warheads and was working on developing VX, a deadlier nerve agent that resists breaking down in the environment.

By early in the last decade, some weapons experts ranked Syria’s chemical stockpile as probably the largest in the world, consisting of tens of tons of highly lethal chemical agents and hundreds of Scud missiles as well as lesser rockets, artillery rockets and bomblets for delivering the poisons.