Arkansas

from the article:

The artillery shells photographed on Oct. 9 and included in the recent Amnesty report bear markings that indicate they were assembled and loaded at the Pine Bluff Arsenal in September 1991. According to a U.S. Army document cataloging types of ammunition, each shell contains 116 felt wedges soaked with white phosphorus. After being fired into the air, a charge within the shell disburses the wedges, which then ignite upon contact with the air and generate thick, white smoke that flows downward as they fall to the ground. When deployed by an artillery weapon, such as a howitzer, they can cover an area as large as a football field

As long as the fuse is set properly and ignites at the proper altitude, all the WP will have burnt off by the time the felt wedges hit the ground. The resulting thick smoke will make you cough but it is not WP, and it will not burn,