A World War I-era mortar shell that likely contains phosgene, a choking agent, was found June 27 on a firing range impact area at Schofield Barracks, the Army said yesterday.

The discovery followed the destruction last year of 71 chemical weapons at Schofield — the largest concentration of unexploded, or "dud," chemical weapons ever found in the United States.

Workers found the 17-inch-long mortar shell while conducting ground excavations at the firing range, which is being converted to a Stryker vehicle "Battle Area Complex."

It is the same area where the 71 previous chemical weapons were found, officials said.

Initial identification by Schofield Barracks Explosive Ordnance Disposal personnel concluded that the round is a WWI-era liquid-filled Stokes mortar.

On Tuesday, a team of specialists from Aberdeen Proving Ground, Md., conducted a non-destructive assessment using a portable isotopic neutron spectroscopy device to determine the contents.

Tests indicate the round likely contains phosgene, an industrial chemical used to make pesticides and plastics which was also used as a choking agent in WWI, the Army said.

"The health and safety of those who live and work on Schofield Barracks, as well as in our surrounding communities is our primary concern," said Col. Matthew T. Margotta, commander, U.S. Army Garrison, Hawai'i. "We have the technical experts on site to assist in the safe handling and storage of this round until its disposal, and are coordinating with state and local officials to ensure appropriate safety procedures are implemented."

The round found on June 27 was secured in an ammunition containment facility pending destruction, the Army said.

The Army last year said it did not have an explanation for the large number of chemical weapons found at Schofield.

The phosgene and chloropicrin rounds, manufactured from World War I on, were stockpiled through World War II.

The chemical weapons, which included several-foot-tall 155 mm artillery shells, were individually destroyed between April and August of last year in a transportable detonation chamber. The cleanup effort cost $7 million.