FARGO, N.D. — Tribal leaders on the Spirit Lake Indian Reservation in northeastern North Dakota are requiring residents to wear masks to help slow the spread of the coronavirus.

It is a rare move in a state where face coverings have not been mandated despite a steady increase in overall cases.

The reservation is primarily in Benson County, which according to the COVID Tracking Project has seen the state’s most new cases per capita in the last two weeks.

The administrator for the Lake Region District Health Unit says the increases in Benson are “basically coming from Spirit Lake,” although it’s not yet clear why. Officials hope a mass testing scheduled for Friday will provide more clues.