A visitor to Yellowstone National Park was killed after a fatal encounter with a grizzly bear on Wednesday morning.

The incident occurred on the Wapiti Lake trail, which is located east of the Grand Loop Road south of Canyon Village.

The husband and wife couple had traveled about a mile and a half when they startled a grizzly sow with cubs. In an apparent attempt to defend a perceived threat to her cubs, the bear attacked and fatally wounded the man, the national park authority said in a statement.

“It is extremely unfortunate that this couple’s trip into the Yellowstone backcountry has ended in tragedy,” said Dan Wenk, Superintendent of Yellowstone National Park. “Our heart goes out to the family and friends of the victim as they work to cope with their loss.”

Though such attacks are extremely rare, when a human encounters a protective sow with cubs surprising a grizzly bear, deadly attacks by bears mostly happen.

“No visitors were injured by bears in the park in 2010. This is the first bear-caused human fatality in Yellowstone since 1986,” the authority said.

Wapiti Lake trailhead is one of the access points to the Pelican Valley area, which is known for major bear activity.

Following the incident, Yellowstone National Park officials have closed all trails and backcountry campsites in the region until further notice.

Officials advised park visitors “to stay on designated trails, hike in groups of three or more people, and to be alert for bears and make noise in blind spots.”