;WASHINGTON (AP) – The U.S. Army Corps of Engineers is beginning a major dig for World War I chemical munitions near American University and the South Korean Ambassador’s residence.

It is the fourth excavation on or near the Washington, D.C., campus in nearly 15 years, since the discovery of disposal pits from the Army’s former chemical warfare station. Officials say the artillerary could contain toxic agents such as mustard gas.

The station was used for developing and testing weapons.

The latest site, a home between the ambassador’s house and the former university president’s home, is the “last known burial pit,” said Dan Noble, project manager for the cleanup.

A tan fabric “vapor containment cover” over the structure is designed to prevent the release of any toxic fumes, and air filters in the back yard are designed to capture any chemical vapors.

Work officially began Monday, but no munitions had been located by Thursday, project engineers said. The $8 million excavation is scheduled to last 14 weeks.

American University President Neil Kerwin notified the campus of the dig in a Sept. 27 memo. He wrote that tests show the possible presence of chemical munitions, including mustard gas and arsine, a flammable gas that is considered more volatile. Kerwin called a chemical release “highly unlikely.”