Rescue workers are still searching for two hikers who went missing late Saturday.

One of those missing hikers is the wife of the founder of the Merrell Boot Company, according to the Associated Press. Chief Ranger Matt Van Dzura says an alert was sent out Saturday evening, and the park service called in the Arizona Department of Public Safety to investigate.

“They encountered two people out of a party of four who were attempting to cross to Pete’s Creek. Two of their members were swept down the creek,” Van Dzura said.

DPS returned the first two hikers safely to Grand Canyon Village. Van Dzura says they sent out helicopters and ground searchers on Sunday but had no luck. He says rescue efforts increased on Monday with additional ground workers, air missions and a drone.

Van Dzura says hiking and backpacking at the Grand Canyon aren’t easy, but incidents like these are rare. He says anyone planning a hike should research routes and safety measures before heading out.