WASHINGTON (REUTERS) - Federal authorities are investigating a possible pesticide poisoning that left two Delaware boys in comas after a family vacation to the US Virgin Islands last month, officials confirmed on Monday.

The boys and their parents, Stephen Esmond and Theresa Devine, had to be airlifted to hospital after falling ill during a stay at the luxury Sirenusa resort in Cruz Bay, St John, according to the family's lawyer, James Maron.

Two weeks after the incident the boys, both minors, remain in critical condition due to "this unthinkable tragedy", Mr Maron said in a statement. Their mother has been released from hospital and Esmond "is improving and stable", he said.

The US Environmental Protection Agency said on Monday that preliminary results confirmed the presence at their rented unit of methyl bromide, a dangerous pesticide that may have caused the illnesses.

"EPA took action and the agency has conducted sampling at the site," the agency said in a statement.

The use of methyl bromide in the United States is restricted due to its acute toxicity and exposure can cause respiratory problems and damage to the central nervous system, the EPA said.

"Only certified applicators are allowed to use it in certain agricultural settings and it is not authorised for use in indoor dwellings," it added.

The rental unit was directly above another unit that had been fumigated by Terminix, owned by Memphis-based ServiceMaster Global Holdings Inc, a company spokesman confirmed.

In a statement, Terminix said the company was praying for the family while also "cooperating with authorities in their investigation, and we're conducting our own thorough internal investigation".

According to its website Terminix is the leading provider of termite and pest control services in the United States.

The EPA said it is working with the Virgin Islands government and the US Department of Justice to investigate whether any environmental regulations or laws were violated.

The US Department of Justice has initiated a criminal investigation into the matter, according to a US Securities and Exchange Commission filing on March 30 by ServiceMaster.

"The family is confident that the responsible parties will be brought to justice and held accountable," Mr Maron said.