Federal environmental officials continue to investigate a Delaware family's serious illness after being exposed to a toxic chemical during their vacation at a Virgin Islands resort.

U.S. Environmental Protection Agency officials say the agency is investigating the use of the pesticide methyl bromide at the Sirenusa Condominium Resort in Cruz Bay, St. John. The chemical is prohibited for residential settings.

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

A family spokesman has said Steve Esmond, wife Theresa Devine and their two teenage sons are being treated at hospitals in the United States. The sons are in critical condition.

The EPA has said Terminix applied the chemical at the resort.