CALAIS, Vt. — Federal and state environmental teams on Wednesday investigated the extent of health risks related to damaged sewage and water treatment plants in more than a dozen Vermont towns where flash flooding has left thousands of people without electricity or potable water since Sunday.

Engineers from the federal Environmental Protection Agency and the state’s Department of Environmental Conservation visited several areas that had been cut off to assess the hazards, officials said. The teams were trying to determine the extent of damage to sewage and water plants in at least 13 towns, including chemical and other hazardous material spills and leaks, said Justin Johnson, deputy commissioner of the environmental department.

Three days after remnants of the tropical storm that began as Hurricane Irene spawned torrential downpours in areas rarely bothered by flooding, access in and around much of southern Vermont remained so difficult that officials were unsure how many facilities had been contaminated because they had been unable to send out inspectors until Wednesday.

“We have 13 towns on ‘boil water’ notice, where we know there has been some level of damage, but we don’t have an exact count right now,” Mr. Johnson said, adding that he expected the number to grow.