GREENWICH, NY — (CNN) A winter tradition turned tragic when two boys building a fort were trapped after a snow bank collapsed on them.

Police in Greenwich, New York, said Joshua Demarest, 13, died Tuesday night in the collapse. His 12-year-old friend survived and is recovering from hypothermia.

The boys had crawled inside the snow bank to build a fort, police said.

The pair were reported missing when they didn’t return home at nightfall, according to Greenwich Police. After school, they had dressed in their winter clothes and taken off with their sleds.

Searchers were led to a snow bank in a nearby parking lot by tracks — and then spotted a sled sticking out from the bank. Greenwich Police Chief George Bell said officers started a frantic digging operation, removing seven tons of snow. Joshua was found and taken by ambulance to a local hospital, where he died. The other boy was found alive.

The cause of the accident is under investigation, but police believe a public works crew may have dumped more snow on the structure, causing the collapse.

“We know (the boys) burrowed into the backside of the snow bank to make a fort, like all kids do,” Bell said.

Snow plows work the area where the boys were playing and dump cleared sidewalk snow at the location, authorities said.

Public works Superintendent Leo Flynn told CNN affiliate WTEN that his workers “are devastated.”

The boys, who were described as best friends, lived on the same block and attended the same Greenwich middle school.

The-CNN-Wire ™ & © 2016 Cable News Network, Inc., a Time Warner Company. All rights reserved. (PHOTO: WTEN via CNN)