FRESNO, Calif. -- A medical helicopter carrying a patient to a hospital crashed amid heavy rain and fog in a rural area of central California, killing all four people aboard, officials said.

@kerncountyfire says 4 dead in #medevac helicopter crash. Our thoughts and prayers are with the victims' families & friends #LODD #heroes — Malibu Search Rescue (@MalibuSAR) December 11, 2015

The SkyLife air ambulance had a pilot, a nurse, a paramedic and a patient aboard when it went down in a remote field durinf its planned 50-mile trip Thursday night, American Ambulance president Todd Valeri said at a news conference. The crash happened about halfway into the trip, reports CBS Bakersfield affiliate KBAK-TV.

The Bell helicopter was headed from Porterville Municipal Airport south to San Joaquin Community Hospital in Bakersfield when it crashed, Kern County fire officials said. Officials did not say why the patient was being taken to the hospital, and the names of the dead are being withheld until family members can be informed.

There was dense fog and heavy rain in the area and it wasn't clear whether that caused the crash, but "weather conditions are always a factor," Valeri said.

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The weather also made it difficult to find and reach the site, and rescue crews didn't get there for more than two hours after the helicopter went missing.

The crew notified flight dispatch that it was leaving Porterville at 6:52 p.m., according to Dan Lynch, EMS Director for Fresno, Kings, Madera and Tulare counties.

At 7:05 p.m., dispatch tried to raise the helicopter for a routine check in, but got no response, Lynch said. After checking with airport towers in Fresno and Bakersfield, the dispatch center notified Kern County authorities that the aircraft was missing and provided its last GPS reading.

A sheriff's helicopter crew reported finding the debris field on private property near the town of McFarland around 8:35 p.m. Rescuers reached the crash site around 10 p.m. and confirmed the fatalities.

The helicopter was staffed by a "seasoned crew," Lynch said. "They had been a team for quite some time".

The Federal Aviation Administration and the National Transportation Safety Board would investigate, officials said.

Air Ambulance's Skylife Air Medical service operates three helicopters out of the Fresno and Visalia airports. Valeri said that SkyLife has never had an aircraft crash since it partnered with Rogers Helicopters Inc. in 1991. The air ambulance transports about 1,000 patients a year, he said.