OCEAN SPINGS, Miss. (AP) — Authorities have indefinitely closed about 30 acres on Horn island in the Gulf Islands National Seashore for a cleanup, saying broken tiles containing asbestos had been found at the former biological testing site for the U.S. Army.

Dan Brown, superintendent of the Gulf Islands National Seashore, said at a news conference Monday that workers conducting an inspection on Horn Island in June in the wake of the 2010 Gulf of Mexico oil spill found asbestos tiles scattered around about one acre. He says a test also came back positive for mustard gas in one area of the site.

The National Park Service said it confirmed finding broken asbestos tiles at the former military test site, which was decommissioned in the 1960s.