The U.S. Army Corps of Engineers has halted its search for World War I chemical weapons at an excavation pit near American University while it assesses whether safety procedures are adequate for munitions that are being uncovered, officials said.

The review was prompted by the preliminary analysis of an artillery round discovered about three weeks ago in the pit in an affluent area of Northwest Washington.

A metal containment structure was built over the pit with blast-resistant walls, along with a decontamination tent. The Army's safety procedures assume a worst-case scenario involving the release of arsine, a toxic chemical agent from a 75mm artillery round that was not configured to explode.

But Corps officials said they do not know whether the artillery round recently discovered is explosively configured. The round is being analyzed at a Corps facility on federal land near Sibley Hospital, said Ed Hughes, the Corps' Spring Valley program manager.

The excavation began in October. It is the fourth dig on or near the campus in nearly 15 years following the discovery of disposal pits from the Army's former chemical warfare testing facility at the university.

The area behind the campus was mostly farmland during World War I, but developers later unknowingly built multimillion-dollar homes over the munitions pits. A contractor at one home found unexploded mortar and artillery shells in 1993, leading to an evacuation and subsequent excavations.

None of the munitions excavated in the area since chemical weapons were discovered has been configured to explode, said Dan Noble, a Corps official overseeing the excavation.