Just a couple of items to add to the discussion, if they have been covered already my apologies, but I haven't the boss-dodging skills to read all 189 previous posts.
The M120 Mortar (at least the M120/M121 version used by the US Army) weighs about 315lbs and is transportable by trailer or Medium sized vehicle (think Hummer). It fires a 30 lb shell (weight includes cartridge, propellant, and warhead) approximately 2-7km (1.5-5 Miles) at a fire rate of 6 per minute.
Mustard Gas (there are several varieties, the most popular being Sulfur Mustard) is a powerful irritant and blistering agent that damamges the skin, eyes and respiratory track, it is usually not fatal. when sulfur mustard was used during WWI, it killed fewer than 5% of the people who were exposed and received medical care. Sulfur Mustard can damage DNA, however (from CDC)
Not knowing the exact type of Mustard Gas but using sulfur mustard as our basis (it was the first mentioned on the sites I checked and given that it could be produced in quantity in 1916, it would be the most likely for a less-developed power to produce) we have a weapon that is designed more to disable enemy soldiers and force them into protective gear, where their fighting ability is lessened. As a terror weapon, well, given that a terrorist could get both shells and mortar into the US, set it up within 5 miles of a target, and successfully fire all 36 rounds to date discovered, they would get panic and confusion and injury, but no infrastructure damage and a limited number of dead (medical help has improved slightly since 1916). Weapon-sure Mass--if you mean 9-11 numbers of ~3,000, doubtful. Destruction--no meaningful physical destruction and a low kill ratio doesn't sound like the type of destruction a committed terrorist would desire.
A question I posed earlier remains: Does the discovery of 36 shells designed for limited battlefield use and of limited fatal consequences that were buried for 15 years equal 500+ American deaths, $100B+ expenditure, the destabilization of a nation, and worsened relations with the other major democracies of the world? Maybe so, but it would be nice to have a report on the facilites needed to produce the items, the storage and maintenance of the weapon systems, and the paper trail of where the raw material came from.
Still, tempus fugit, and another 20 years should tell the tail--we humans are in such a rush sometimes.