Emergency crews ceased searching for a lost scuba diver at Canyon Ferry Reservoir at about 8:40 p.m. Monday.
"We were able to continue the search after the weather passed, but have not found him," said Lewis and Clark County Sheriff's Capt. Kevin Wright. "At this point, we are calling it a recovery."
Efforts will resume at about 8 a.m. Tuesday.
Crews had been searching the northwest end of Canyon Ferry Reservoir since Monday afternoon. Rescue efforts did cease at one point to wait out inclement weather but later resumed and continued until the sun began to set.
Wright said two divers were in the water when one had an equipment malfunction. The other diver tried to help but the diver with the equipment malfunction "quickly went under."
The diver had not been seen since about 1:30 p.m. Crews are searching an area near Cemetery Island on the reservoir's northwest end and launching from the Yacht Basin Marina.
The diver who attempted to assist was taken to shore and transported to St. Peter's Health, Wright said. The diver was initially reported to have had trouble breathing but appeared to be breathing normally in the ambulance, he said.
Emergency responders from the sheriff's office, Lewis and Clark Search and Rescue, Tri-Lakes Fire and Montana Fish, Wildlife & Parks are participating in the search.