A fire broke out Saturday night at a house in Reston while the occupants were inside, the Fairfax County Fire Department said.

Occupants escaped safely from the house in the 11900 block of Gray Squirrel Lane, said deputy fire chief William Garrett.

He said firefighters found “heavy fire” in the back of the house, which extended inside.

The cause was under investigation, he said.

The site is a cul de sac east of the Fairfax County Parkway.