The search for a missing 24-year-old man is entering its fourth day after he ventured out on a solo hike in a California canyon.

Ian Roberts, of Roseville, Calif., was last seen at 9:45 a.m. local time on Friday, Aug. 21, the Placer County Sheriff’s Office said in an initial alert. After he failed to return home that night, he was reported missing the next morning, on Saturday, Aug. 22, just before 5 a.m.

With the help of Roberts’ family, deputies were able to find his vehicle at a skate park in Auburn, Calif.

“Investigators believe Ian parked at Overlook Park before hiking down into the American River Canyon area on Friday August 21st,” officials said at the time. “Search efforts have been primarily focused in this area.”

With the help of helicopters, drones, and K9 search and rescue teams, officials scoured the Auburn State Recreation Area on Saturday, before darkness descended.

By Sunday, Aug. 23, about 100 search and rescue personnel from various agencies were assisting in the search, as well as a marine unit that looked for Roberts along the banks of the American River.

The same number of crew members were out on Monday, Aug. 24, authorities said.

“Search crews are utilizing both traditional and fixed-wing drones, while the Placer County Sheriff’s Office Marine Unit is searching the river and shoreline areas,” the sheriff’s office said.

They added that Monday’s conditions were “challenging,” with temperatures reaching the mid-90s.

A spokesperson for the Placer County Sheriff’s Office confirms to PEOPLE that Roberts remains missing as of Tuesday, Aug. 25. Another search effort was set to begin that morning.

“We are incredibly grateful to the many search and rescue professionals and volunteers who have traveled from throughout the region,” the sheriff’s office said, “and are working to help bring Ian home.”