Neither the FBI nor the CIA could get to the bottom of this.

The mysteries of aviation have always been a fascinating topic to explore and have resulted in some of the most fascinating documentaries to grace our television screens.

The disappearance of Malaysia Airlines MH370 in 2014 remains unsolved, and a Netflix series explores how a plane carrying 239 passengers disappeared without a trace.

One unsolved case dates back even further, to the 2003 theft of a plane from an Angola airport.

On May 25, 2003, two men boarded a Boeing 727 parked at Luanda April Airport and flew southwest across the Atlantic Ocean.

At the time, commercial aircraft were not equipped with the same tracking technology as now.

Not to mention this happened only two years after 9/11, so everyone was on high alert.

Two thieves entered the cockpit and turned off the lights and transponder shortly after taking off from the airport.

One of the men is a U.S. citizen and a private pilot, and the other is from Angola.

Since that day, neither the people nor the plane have been seen, and the mystery has not only baffled the public, but CIA and FBI investigators remain baffled as to what happened.

One of the leading theories about what happened involves legal issues and dodgy dealings.

The aircraft was being transferred from one airline to another when it was stolen.

There were reportedly numerous problems with the aircraft leases, as well as other financial issues such as missed payments and case files.

After all this information came to light, many speculated that the two men who carried out the robbery were hired to do so in order to avoid legal trouble and commit fraud.

More than two decades later, this is the most commonly proposed explanation, but no one knows for sure.

Although legal agencies have withdrawn from the case in recent years due to a lack of new information, the case has never been officially closed and remains a mystery.

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