The day before a brand new Lion Air jet crashed into the sea shortly after taking off from Indonesia's capital city last fall -- likely due to an equipment malfunction -- an off-duty pilot reportedly helped save the aircraft when it began to dive.

The extra pilot was on the flight from Bali to Jakarta and was seated in the cockpit jumpseat when the crew of the Boeing 737 Max 8 struggled for control of the aircraft, Bloomberg reported on Tuesday.

During the flight, the jet displayed unusual variations in altitude and airspeed in its first several minutes, Reuters previously reported. Some of those variations included an 875-foot drop over 27 seconds when the plane would typically be ascending, before stabilizing and flying on to Jakarta.

As the jetliner was in a dive, the extra pilot figured out what was wrong and told the crew how to disable a malfunctioning flight-control system to save the aircraft, two people familiar with Indonesia's investigation told Bloomberg.

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The crew was told to cut power to the motor causing the plane's nose to dive down, which is part of a checklist that all pilots are required to memorize, according to Bloomberg.

Hours later with a different crew on board, the same aircraft crashed into the Java Sea after takeoff, killing all 189 aboard.

Officials investigating the Lion Air crash previously said they were looking into the plane’s anti-stall system, which was engaged and repeatedly pushing down the aircraft’s nose prior to it crashing into the sea. The Indonesia safety committee report also said the plane had had multiple failures on previous flights and hadn’t been properly repaired.

“All the data and information that we have on the flight and the aircraft have been submitted to the Indonesian NTSC. We can’t provide additional comment at this stage due to the ongoing investigation on the accident,” Lion Air spokesman Danang Prihantoro told Bloomberg.

The new information comes as investigators are trying to piece together what caused Ethiopian Airlines Flight 302 to crash earlier this month just minutes after takeoff, killing all 157 people, including eight Americans, aboard.

The French civil aviation investigation bureau, BEA, said Monday that black box data from the Ethiopian Airlines flight showed "clear similarities" to the Lion Air crash. Ethiopian authorities asked BEA for help in extracting and interpreting the crashed plane's black boxes because Ethiopia does not have the necessary expertise and technology.

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The U.S. Federal Aviation Administration previously said satellite-based tracking data showed that the movements of the Ethiopian Airlines flight were similar to those of the Lion Air plane. In both incidents, the planes flew with erratic altitude changes that could indicate the pilots struggled to control the aircraft. Shortly after their takeoffs, both crews tried to return to the airports but crashed.

The Federal Aviation Administration last Wednesday announced Boeing 737 Max 8 and 737 Max 9 models were being temporarily grounded in the U.S. "as a result of the data gathering process and new evidence collected at the [Ethiopia crash] site and analyzed."

The U.S. joined several other countries in grounding the aircraft following the deadly crash.

On Monday, the Associated Press reported that prosecutors are looking into the development of Boeing's 737 Max jets, including how the company was regulated by the Federal Aviation Administration.

A federal grand jury in Washington sent a subpoena to someone involved in the plane's development seeking emails, messages and other communications, the person briefed on the matter told the AP.

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Spokesmen for the Justice Department and the inspector general said Monday they could neither confirm nor deny the existence of any inquiries, while the FAA would not comment.

"Boeing does not respond to or comment on questions concerning legal matters, whether internal, litigation or governmental inquiries," Boeing spokesman Charles Bickers told the AP in an email.

The company late Monday issued an open letter from its CEO, Dennis Muilenburg, addressed to airlines, passengers and the aviation community.

Muilenburg did not refer to the reports of the Justice Department probe, but stressed Boeing is taking actions to ensure its 737 Max jets are safe, including a software update and offer related pilot training for the 737 Max to "address concerns" that arose in the aftermath of October's Lion Air crash. The planes' new flight-control software -- which automatically pushes the plane's nose down when a single sensor detects the nose is pointed too high, indicating the possibility the aircraft could stall -- is suspected of playing a role in the crashes.

Fox News' Katherine Lam and The Associated Press contributed to this report.