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A criminal investigation is underway after a family of four was sickened following a fumigation at a Virgin Islands resort, according to the U.S. Environmental Protection Agency.

The EPA was notified on March 20 that a family staying at Sirenusa Resort in Cruz Bay in St. John, became ill and had been hospitalized following a fumigation in a room at the resort two days earlier, the agency said in a statement released March 23.

The EPA is investigating whether the chemical that was used during the fumigation was methyl bromide — a toxic chemical that is restricted in the U.S., the statement said.

Inhalation of the odorless chemical "may be fatal or cause serious acute illness or delayed lung or nervous system injury," according to the EPA.

The Delaware couple and their two teenage sons were hospitalized in Philadelphia. Stephen Esmond, the father, is the headmaster of a Delaware middle school and his wife, Dr. Theresa Devine, is a dentist, according to NBC Philadelphia.

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Devine had been discharged as of Monday, and Esmond is in stable condition, according to the family's attorney and spokesman, James J. Maron. The two teenage boys are still in critical condition, but the family is "hopeful for a full recovery," Maron said.

"The EPA is continuing to work with the U.S. Virgin Islands government and others to gather information and will ensure that appropriate steps are taken if it determines any environment regulations or laws were violated," the EPA said.

Mary Mears, an EPA spokeswoman, told NBC News that the investigation was still being conducted Monday.

Sea Glass Vacations, the rental agency for Sirenusa, said in a statement that the room below where the family stayed from March 14 to March 22 had been "recently" fumigated by the extermination company, Terminix.

"Sea Glass Vacations does not treat the units it manages for pests but instead relies on licensed professionals for pest control services," the statement said.

Terminix said in a statement that the company is cooperating with investigators — and conducting one of their own. "We're committed to performing all work we undertake in a way that is safe for our employees, customers and the public," Terminix spokesman Pete Tosches said.

The United States Department of Justice has also opened a criminal investigation into the case, according to a Securities and Exchange Commission report filed by ServiceMaster Global Holdings, Inc., the parent company of Terminix.

"The Esmond family thanks everyone for their support and concern for their recovery from this unthinkable tragedy. The family is confident that the responsible parties will be brought to justice and held accountable,” Maron, the family attorney, said.

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