Glacier National Park rangers took the unusual step of closing three of the park's most-used trails late Sunday after a grizzly bear or bears near Granite Park Chalet and campground showed unusual aggression when encountering hikers.

All access to the Granite Park area is closed until further notice, including the Highline, Loop and Swiftcurrent (from Swiftcurrent Pass to Granite Park Chalet) trails.

In a written statement, park officials said rangers will hike to the area on Monday morning to observe be a behavior and conduct hazing, as appropriate.

The Granite Park backcountry campsite will be closed to campers arriving Monday.

Guests planning to hike to Granite Park Chalet with reservations for Monday night should call the Granite Park Chalet office at 888-345-2649 for more information and updates. The park does not expect that any trail status updates will be available before Monday afternoon.

People currently in the Granite Park region departing Monday morning will be permitted to hike out the Loop or Swiftcurrent Pass trails, but not the Highline Trail.

The park expects that the trails will be closed at minimum until park staff can evaluate the area, and may be closed an indeterminate period of time afterwards, depending on their findings.

Park staff who live in the Granite Park area have been monitoring grizzly bears frequenting the area and on Sunday received several firsthand reports from visitors who had encounters with a bear or bears along the trail within the general area of the campground and the chalet.

"The bear or bears exhibited behavior consistent with being disturbed and frustrated by human presence," Glacier officials said. "Bears can respond aggressively in defense of themselves, a food source, or cubs."

“We appreciate the public’s patience while we evaluate this situation,” said Glacier National Park Superintendent Jeff Mow. “The park has a proactive bear management program, and we take reports of aggressive bear behavior very seriously.”

People can monitor trail status by visiting the park’s Trail Status webpage.