One of two black box recorders was found amidst the wreckage of a China Eastern Airlines flight, giving investigators hope it will shed light on why the passenger jet plummeted from the sky and crashed into a mountainside in southern China, authorities said Wednesday.

None of the 123 passengers and nine crew members aboard Flight 5735 survived when it crashed in the Guangxi region.

An official from the Civil Aviation Administration of China confirmed Wednesday that one of the recording devices was located. The official said the protective exterior of the box was seriously damaged.

During a news conference Wednesday, Chinese authorities said that they believe the recovered device is the cockpit voice recorder. It was found in the main impact point of the crash and has been sent to Beijing for repair and analysis, officials said.

Authorities publicly acknowledged for the first time that human remains were found at the crash site.

Investigators are still searching for the second recorder that stores flight data, including airspeed, altitude and wing flap positions.

Early data shows the Boeing 737-800 plunged from 29,000 feet to 8,000 feet, leveled off and then went into a freefall, exploding into a fireball that was seen and filmed by people nearby. One video showed the plane nose-diving into the ground.

“What that cockpit voice recorder is going to tell us is what were the pilots saying to each other. What were they doing? It will mean the difference between being able to say, okay, obviously, they had a major emergency they were fighting all the way down, or maybe there was only one person in the cockpit. We don’t know,” said ABC News contributor John Nance, a former commercial pilot.

The plane crashed after taking off from Kunming, the capital of China’s Yunnan province. The flight was headed to Guangzhou, a port city northwest of Hong Kong, Chinese officials said.

Air traffic controllers made repeated attempts to radio the flight crew when they noticed the aircraft’s rapid descent but were unable to restore communications with the crew before the crash, Chinese officials said.

U.S. intelligence doesn’t have a clear theory on what led to the plane crashing. A source tells ABC News they aren’t ruling anything out, including a possible intentional downing.

“Having an airliner impact the ground as fast as this is going, almost to the speed of sound, means that it’s going to pulverize the airplane and everything in it,” Nance said. “However, the cockpit voice recorder and the flight data recorder are in the tail of the airplane, which slows down considerably by the time they reach the ground. Consequently, even though the box is mangled, the chip inside it can withstand almost 100 G’s, so that should be OK.”

As a precaution, China Eastern Airline grounded its fleet of Boeing 737-800s on Wednesday.

Members of the U.S. National Safety Transportation Board, the Federal Aviation Administration, Boeing and engine-maker CFM International are all joining the probe.

ABC News’ Amanda Maile contributed to this report.