That's what investigators are trying to find out.
The crew of the E.S.S. Pursuit picked up at least 10 old military canisters as they were dragging the ocean floor clamming just south of Long Island Sunday.
New Bedford fisherman Kevin O'Sullivan and another crew member handled one of the canisters as they threw them back into the ocean.
It broke open and exposed a fisherman named Costa, causing blistering on his arm and leg.
"He experienced immediate pain," said O'Sullivan, who also had shortness of breath.
Both men were taken to the hospital, while two more crew members were transported Monday night when the vessel returned to New Bedford.
Now it's quarantined in the harbor.
"There's a 500-foot radius around that ship that no one can go to, except us," said Captain Kevin Morris of the Marine Strike Team.
"My eyes are quite sore; I wash my eyes every few minutes. There's burning on my face, cheeks and hands," said E.S.S. Pursuit captain Kieran Kelly, who has refused to leave the boat during the investigation.
While investigators have narrowed down the chemical agent, they've yet to officially identify it.
As a precaution, the Coast Guard has marked the area south of Long Island where the canisters were dumped.
O'Sullivan told WBZ the canisters had the date "1914" or "1918" on it.
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