BOSTON - State and federal officials worked Tuesday to decontaminate an Atlantic City-based clam boat anchored in isolation off Massachusetts after it dredged up old munitions laced with mustard gas, severely sickening a crewman.

The Coast Guard was trying to locate the two military shells, which the crew of the ESS Pursuit tossed back into about 60 feet of water about 45 miles south of Long Island, said Coast Guard Petty Officer James Rhodes. Finding them would be difficult, he acknowledged.

The military used the ocean as a dumping ground for munitions from after World War II through 1970. The shells that came aboard the 145-foot dragger were in a haul of clams. The area is in a known munitions dumping zone and is on charts, Rhodes said.

A National Guard team boarded the vessel on Tuesday to test for contamination while the Coast Guard worked to secure the ship off New Bedford so that it could be decontaminated.

The boat returned to New Bedford early Monday after one of its six crewmen reported blistering and shortness of breath. Hours later, another crewman came ashore after he reported feeling light-headed. He was examined and released. Two others left the boat late Monday, with one reporting nose and eye irritation.

The most seriously injured crewman, whom officials did not identify, had painful blisters about three-quarters of an inch high on an arm and a leg, said Edward Boyer, a toxicologist who is treating him at UMass Memorial Medical Center in Worcester.

Boyer said blood and urine tests confirmed exposure to mustard gas, used most frequently during World War I.

"It literally pulls the top of the skin off the layer underneath it," he said. His patient was "handling it very well," he said.

The Defense Department began dumping chemical and conventional munitions at sea after World War II. The military says it stopped in 1970, and two years later Congress banned waste disposal in oceans, including chemical weapons.

Officials say it's impossible to know how much and what weapons have been dumped because of incomplete records. A 2001 Army report found 74 instances of ocean disposal - 32 off U.S. shores and 42 off foreign coasts.

For example, in 1967 the Army dumped 4,577 one-ton containers of a mustard agent and 7,380 sarin rockets off the New Jersey shore, according to Army records.

Only some dumps were mapped, and chemical munitions have been found in areas they weren't supposed to have been dumped, such as off Hawaii, said Craig Williams of the Chemical Weapons Working Group, a Kentucky-based organization.

Mustard gas can be deadly if it's used as an aerosol and inhaled.