BOSTON – State and federal officials worked Tuesday to decontaminate a 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 tossed overboard in about 60 feet of water roughly 45 miles south of Long Island, said Coast Guard Petty Officer James Rhodes. He acknowledged that finding the shells will be difficult.

The military used the ocean as a dumping ground for munitions from after World War II through 1970. While the tons of old chemical weapons beneath U.S. waters present a danger to fishermen, experts don’t believe they are a possible source of weapons for terrorists.

The two shells that came aboard the Atlantic City, N.J.-based vessel Sunday 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, the ESS Pursuit, on Tuesday to test for contamination, while the Coast Guard worked to secure the ship in waters off New Bedford so that it can be moored and decontaminated. The captain and first mate have declined to leave the 145-foot dragger, fearing it could run aground.

The boat had returned to New Bedford early Monday after one of its six crewmen reported blistering and shortness of breath.

Hours later, another crewman was brought ashore after he reported feeling lightheaded. He was examined and released. Two other crewmen 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 Dr. Edward Boyer, a toxicologist who is treating the man at UMass Memorial Medical Center in Worcester. The patient was “handling it very well,” he said. Boyer said.