BILLINGS, Mont. (AP) — For the second time this winter, someone broke into a holding pen for Yellowstone National Park bison, allowing the escape of 73 animals captured to prevent the spread of a wildlife disease.

Park officials said the Stephens Creek pen was opened Wednesday night or early Thursday morning.

Most of the 73 wild bison that escaped stayed nearby and eventually returned to the pen. Fifty-two bison escaped in a similar incident last month.

Bison captured as they leave the park during winter migrations are slaughtered or held in quarantine for possible relocation later.

Park Superintendent Dan Wenk said whoever broke into the pen was threatening the success of the quarantine program and ensuring more bison will be sent to slaughter.