WASHINGTON (AP) – The U.S. Army Corps of Engineers is halting its search for World War I-era chemical weapons in a Washington, D.C., neighborhood after workers found an open glass flask containing traces of the chemical agent mustard.

The discovery was made last week at a vacant home behind American University, where chemical weapons were developed and tested during World War I.

Army Corps project manager Dan Noble says the mustard agent was uncovered two feet below ground but the Corps is confident there was no public exposure. Eight workers underwent precautionary tests but didn’t show symptoms.

During World War I, mustard gas was used to cause blisters, temporary blindness, breathing problems and vomiting and could be fatal in high doses.

Noble says safety procedures will be reviewed before the excavation continues.