GRAND CANYON NATIONAL PARK, Ariz -- Authorities are scaling their search area for a Surprise man reported missing on a hiking trip at Grand Canyon National Park since last week.

Park rangers said Robert A. Williams, 69, was last seen one week ago outside the park's backcountry information center and reported overdue Tuesday from a trip to the park.

On Wednesday, park rangers found Williams' car parked behind the visitor's center on the canyon's South Rim.

So far, the search has proved futile.

"As a result, a decision has been made to begin scaling back search operations," a news release said. "This decision was based on the totality of circumstances including the rugged and steep terrain, the length of time that Mr. Williams has been missing and the number of hikers and searchers that have passed through the area without finding any indications as to Mr. Williams whereabouts."

Park rangers will continue to use dogs and searchers on foot on a limited basis to examine cliff areas where a fall might have occurred.

Williams is described as 5 feet 5 inches tall and weighs about 165 pounds. He has gray hair and brown eyes. He was last seen carrying a dark green Gregory backpack.

Investigators ask people who hiked in the park over the holiday weekend who might have seen Williams to contact the Grand Canyon Regional Communications Center at 928-638-7805.