A 6-year-old boy died when a fence fell on him at a private school in Northern California.

The incident Thursday appears to have been an accident, San Rafael Police Lt. Dan Fink told the Marin Independent Journal. He said the portable, rolling fence used to allow vehicles in and out of the schoolyard came off its track.

Staff members who witnessed the incident at Mark Day School in San Rafael immediately called 911 Thursday afternoon, school officials said.

The boy was taken to a hospital, where he died of his injuries.

“The death of someone so young is devastating for all of us, and even more so under these circumstances and at this time of year, when we are surrounded by celebrations while we mourn,” Head of School Joe Harvey said in a statement.

“The safety of our students is so important to all of us, and we want to understand how this could have happened,” Harvey said.

Counselors were at the school Friday.