WASHINGTON (AP) -- Three members of a Delaware family remained hospitalized Tuesday as federal officials continued to investigate their exposure to a toxic chemical during a vacation at a Virgin Islands resort.

U.S. Environmental Protection Agency officials said the agency is looking into the use of the pesticide methyl bromide, which is prohibited in residential settings, at the Sirenusa Condominium Resort in Cruz Bay, St. John. The Justice Department also is investigating the company that may be responsible.

The Esmond family of Wilmington, Delaware, was staying at the Caribbean resort when the incident was reported March 20. Paramedics responded after four people became seriously ill.

(Braden Gunem, Thinkstock)

A family spokesman said Steve Esmond, his wife, Theresa Devine, and their two teenage sons are being treated at hospitals in the mainland United States. The sons are in critical condition. Esmond has regained consciousness, but his sons have remained in a coma weeks after the exposure, family spokesman James Maron told The News Journal of Wilmington. Esmond is head of a private middle school in Wilmington, and Devine is a dentist.

Use of the pesticide was confirmed the day after the family became ill, which has helped inform doctors and medical experts on how to treat the family, said Judith Enck, the EPA's regional administrator in New York City, which has jurisdiction over the U.S. Virgin Islands.

"We have confirmed that the problem is indeed methyl bromide," she said. "Methyl bromide is a potent neurotoxin. It's a gas. It can cause convulsions, coma, cognitive deficits, inflammation of the lungs. A lot depends on how much a person is exposed to and for what period of time."

The EPA banned the chemical for residential use in 1984 primarily for health concerns. That ban extends to U.S. territories, including the Virgin Islands, Enck said.

So far the investigation has revealed a certified applicator working for Memphis, Tennessee-based Terminix applied the methyl bromide in the complex while targeting an indoor beetle that consumes wood, Enck said. The company is now under a criminal investigation led by the U.S. Justice Department.

Terminix issued a statement to The Associated Press in March saying the safety of its customers and employees is a top priority.

"As such, we are cooperating fully with local and federal officials to determine the cause of the incident reported in St. John," spokesman Michael Wassmer said in an email. He did not respond to a request for comment Tuesday.

Investigators also learned methyl bromide has been used in the complex before, but it's not clear why the ban was ignored.

"Certified pesticide applicators know this is not approved for indoor residential use," Enck said. "The health effects are quite serious."

Sea Glass Vacations, a rental agent for several units at the resort, has said it was aware the family became ill while staying in the Villa Capri unit from March 14 to March 22. In a written statement, the rental company said the unit below Villa Capri was recently treated for pests by Terminix, but the family's unit itself had not been treated.

Now the EPA is continuing to test and monitor the property and is continuing to investigate the details of the family's exposure, Enck said, "to make sure it never happens again anywhere."

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