A five-year-old boy and his grandmother were seriously hurt after a rotting tree collapsed and fell on them as they lounged in a hammock in the backyard of a Loudoun County home Saturday afternoon.

Both the boy and his grandmother were airlifted to the hospital after the apparent accident, which occurred just before 1:30 p.m. at the home in the 1900 block of East Beech Road in the Sterling area, police said. The boy’s injuries, police said, were critical and life threatening, and the grandmother’s injuries were serious but not life threatening.

Liz Mills, a spokeswoman for the Loudoun County Sheriff’s Office, said the two were lounging in a hammock attached to the tree when it fell on top of both of them. She said police believed the case to be an accident.

“That’s pretty freaky,” Mills said. “Especially on a beautiful day like today, to be out in your backyard, and the next thing you know, a tree is falling on you.”

Mills said the tree — which was 12 to 18 inches in diameter — was old and rotted out from termites and ants.