OCEAN SPRINGS, 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 Monday that one acre of the 2,700-acre island is affected but 30 acres are cordoned off to keep visitors away from the site. He said cleanup could take years.

Brown said 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 said a test also came back positive for mustard gas in one area of the former military test site.

The barrier island, which helps make up the Mississippi Sound, is about 14 miles long, one-half mile wide and encompasses 2,700 acres. It is a popular spot for recreational boating and fishing as well as camping.

Brown said asbestos materials were been identified around old concrete foundations on the north side of Horn Island known as The Chimney, which was operated from late 1943 to 1944 by the Department of War to test biological toxins.

In addition, mustard gas residue has been detected in the sand nearby, Brown said.

The closed area includes the Big Lagoon.

"There is literature that suggests mustard containers were brought into the water. That was a prudent way to store mustard gas at the time because it doesn't mix with water," said Cmdr. Brian L. Cook, environmental and sustainability manager with the Public Health Service in Atlanta.

No canisters of mustard gas have been found on the island.

Cook said the military is in the process of destroying any reserves of the chemical. The National Park Service plans to contact the federal agency that handles site cleanups used at defense sites under a federally funded program, Cook said.

There will be phases to go through, studies to complete and several agencies coordinating with each other before the actual cleanup can occur, he said.

"The magnitude would be at my estimate some number of years to complete," Cook said. "This is not uncommon with hazardous waste sites."

Asbestos was a common building material in the 1940s, used to strengthen concrete. The chimney was located in the area where electricity was generated. There were livestock pens nearby to hold the animals used in the tests, and narrow gauge railroad tracks 7-1/2 miles long serviced the facility.

It was eventually deemed unsuitable, and decommissioned in 1945, Brown said.

The U.S. Army Corps of Engineers out of Mobile was charged with constructing and dismantling the facility. The private environmental contractor hired by the National Park Service this summer did the studies on the chards of ceiling or wall tiles, as well as tested the area for many different types of carcinogens and contaminants.

As a part of those tests, they placed a small umbrella over the top of the sand with sensors at 12 different places. One of those 12 sites picked up the presence of mustard gas, Cook said.

"The vast majority of the island is safe and open to the public," said Brown.

A Pentagon report in November 1993 said Horn Island was one of three sites in Mississippi where nerve agents, mustard gas and other chemical weapons might be buried. An Associated Press article said the Army tested biological weapons from 1943 to 1945 on Horn Island and disposed of 133 German mustard gas bombs there in 1946.

The biological test site was moved from Horn Island to Dunway Proving Ground in Utah.