Four suspects who had been charged with taking part in a $50 million (€47 million) diamond heist at the Brussels airport in 2013 were acquitted by a Belgian appeals court Wednesday.

The court explained its ruling saying "the elements of the investigation are not sufficiently reliable," leaving the last four suspects in the case to walk free.

Although one person has been convicted and sentenced to five years in prison, another 18 people were acquitted in 2018.

Only a small portion of the loot has ever been recovered.

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

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

What do we know about the heist?

The robbery took place in February 2013.

Eight heavily armed men cut a hole in the fence at Brussels airport and walked towards a plane dressed as police officers.

The plane was headed to Switzerland transporting some 120 parcels of gems that had been offloaded from a security vehicle that had brought them from the global diamond hub in Antwerp.

Brandishing their machine guns toward the pilots and security officers, but without hurting anyone, the thieves were able to take the diamonds.

The 29 passengers were completely unaware of what was going on.

The heist took just five minutes, leading some to compare the heist with the Hollywood movie "Ocean's Eleven."

Investigators thought they were close to tracking down the criminals several times. They even carried out an operation across three countries which resulted in the recovery of some of the diamonds.

But it remains unclear if the mastermind behind the operation, or the rest of the loot, will ever be found.

ab/ar (AP, dpa)