It was a Hollywood-esque heist – in fact it even drew comparisons to the film "Ocean's Eleven" for its clean, clinical execution that left no one hurt. Eight masked gunmen in two cars with police markings stole diamonds worth nearly $50 million from the Brussels Airport without a single shot being fired. A decade after the brazen robbery, it looks like a near-perfect crime.

While one person was convicted and sentenced to five years in prison and a small part of the loot – estimated in 2013 to be worth $50 million – was recovered, 18 other people were acquitted in 2018 and four more were acquitted on appeal this week, leaving it unclear whether the mastermind will ever be found.

The Brussels appeals court said in its ruling that "the elements of the investigation are not sufficiently reliable" and that they were insufficient to convict the last four defendants in the case.

On a crisp, wintry evening in 2013, eight robbers dressed in police clothing cut through security fences at Brussels international airport and headed for a Swiss-bound plane where parcels of gems from the nearby Antwerp global diamond hub were being loaded.

The robbers hid in a construction site outside the airport prior to the robbery. They were armed with assault rifles and dressed as police officers. Entering the airport through a hole they created in the airport security fence, they drove on the property with two vehicles, a Mercedes van and an Audi, both of which were black with flashing blue police lights.

They drove straight to the airplane where the gems were being transferred from a Brink's armored van, which had driven from Antwerp, onto the Fokker 100 twin engine jet Swiss Flight LX789, which was headed to Zurich.

The time between the loading procedure and the moment the plane started to move to take off would only have lasted 15 minutes, according to Caroline De Wolf, a spokeswoman for the Antwerp World Diamond Centre. De Wolf stated that the window for opportunity was so small that the perpetrators must have known ahead of time about the transfer procedures and timing.

The robbers stopped the plane, then brandished their guns, stopping the pilots and transport security. The Brussels prosecutor's office described the weapons used as "like Kalashnikovs," most likely the Galil. They loaded 130 bags into their cars and drove off, but left behind some gems in their hurry.

The whole robbery took about three minutes and did not appear to disturb any of the passengers, who did not know that anything had happened until they were told to disembark because the flight had been cancelled. The van believed to be used in the robbery was later found abandoned and burned.

Because of the perfection of the operation, there were immediate suspicions there was help from inside the airport.

Investigators thought they were close to finding the robbers several times, especially three months later when they detained several dozen people in a three-nation sweep and recovered some of the diamonds. But the courts found the evidence unconvincing.

"There were so many elements that were presented as overwhelming evidence, but after a close look appeared to be deformed, and badly interpreted," said defense lawyer Benjamine Bovy, adding that the those looking into the case were too fixed on one scenario only.

"The mountain gave birth to a mouse," Bovy said.