Search crews targeted the dark waters of Prince William Sound again Wednesday as they searched for a medical helicopter that vanished two days ago as it transported a patient.

Poor visibility hampered efforts Tuesday to find the LifeGuard Alaska helicopter in the sound, which is ringed by mountains. A thick cloud cover prevented an aerial inspection of the area where the Eurocopter BK 117 last made contact with operators.

"The weather is bad enough that we can't get into any approaches in Prince William Sound," said Col. Dave Lowell, director of operations for the Alaska National Guard.

The twin-engine aircraft was heading from Cordova to an Anchorage hospital when it vanished in Monday evening with a patient and three others on board. The helicopter crew made a satellite phone call at 5:18 p.m. Monday, but it was not a distress call, said Coast Guard Lt. John McWhite.

After that, the aircraft failed to check in for position updates that are supposed to be issued every 10 minutes, said Providence Alaska Medical Center spokeswoman Becky Hultberg. The trip from Cordova to Anchorage is about 150 miles by air, and it usually takes about 90 minutes, Hultberg said.

The patient, a pilot, a paramedic and a nurse were aboard the helicopter. Their names and the reason for the flight were not released.

The aircraft's last known position was the southeast side of Esther Island, about 75 miles southeast of Anchorage, said officials with the Air National Guard, which was leading the search. A Coast Guard cutter and fishing boats were helping the search effort, listening for signals from the helicopter's emergency beacon.

Around the time of the helicopter's disappearance, a National Weather Service buoy just south of Esther Island recorded sustained winds at 25 mph, with gusts around 50 mph, according to aviation meteorologist Victor Proton. The agency issued advisories alerting pilots about moderate turbulence as well as low visibility.

LifeGuard Alaska is an air ambulance service operated by Providence. The missing helicopter is among two leased from Evergreen Helicopters of Alaska, Inc., which provides the pilots and maintenance. Evergreen was referring calls to the hospital, according to the company's answering service.