WASHINGTON — An investigation is underway to determine what caused a house fire that killed a woman and her 6-year-old grandson in Charles County, Maryland, early Wednesday.

Charles County Fire officials say a fire broke out at about 12:30 a.m. fire at 15101 Poplar Hill Rd. in Waldorf. A woman and her grandson died the blaze. Four people were able to escape the fire without injury after the smoke alarm sounded.

The Charles County Sheriff’s Office identified the victims as 68-year-old Bernice Washington and 6-year-old Zavion Atkins.

“This is certainly a tragedy within the community, and certainly within the fire department community,” says Charles County Fire spokesman Bill Smith.

When 55 Charles County firefighters and mutual aid from Prince George’s County arrived at the double-wide modular home, they found heavy fire conditions.

“We did as much as we could as quickly as we could but it’s very unfortunate circumstance this morning,” Smith says.

“During the investigation, we did determine that a working smoke detector was properly installed and working.”

The home is a total loss.

One firefighter was taken to the hospital for a back injury, but has since been released.

The fire marshal and sheriff’s office are investigating. Foul play is not suspected.

WTOP’s Kristi King contributed to this report.