It took at least nine hard blows from an ax to smash the glass case in Dresden's historic Green Vault. Once the glass shattered, the two masked thieves grabbed 21 priceless diamond-studded artifacts and disappeared.

It was November 25, 2019, and in the space of a few short minutes, some of the world's most valuable historic jewels had vanished.

A trial with the six men accused of carrying out one of the biggest jewel thefts in history has started in Germany on Friday, January 28. But the mystery of what happened to the treasures they are alleged to have stolen endures.

This is the story of a heist that stunned the world -- and the meticulous police work that led to the capture of six members of the family gang that police say are believed to be responsible for it.

Adorned with more than 4,300 diamonds, the treasures stolen from the Green Vault were worth at least 113 million euro ($128 million), according to the state prosecutor's office. However, the director of Dresden's State Art Collection, Marion Ackermann, said their material value doesn't even begin to reflect their "incalculable" historical and cultural importance.

Nearly all the stolen artifacts were made during the rule of Frederick Augustus III, the last Elector of Saxony, who was later known as Frederick Augustus I, the first King of Saxony.

They included a 1780s hat clasp decorated with 15 large and more than 100 small diamonds, as well as a 96-centimeter (38-inch) sword and a scabbard, or sheath, which together contained more than 800 diamonds.

But it wasn't just the immense value of the loot that captured the world's attention, it was the brazenness with which the raid was allegedly carried out.

Roy Ramm, a security consultant and former commander of specialist operations at New Scotland Yard in London, told CNN that crimes like this are increasingly rare.

"Technical security has improved over the years with CCTV alarm systems and all kinds of high-tech protections, so [there is a high] risk of early detection and being actually caught in the act ... you need some inside information and a very, very detailed plan," he said.

According to investigators, four months before the robbery, a suspect went to the city of Magdeburg, 180 miles northwest of Dresden, to collect a dark blue used Audi S6: the future getaway car.

The vehicle had already been deregistered, but the police said the gang went even further in their efforts to disguise its origins, changing its color to silver and leaving only the roof dark.

"What this says to me is that these people planned meticulously; they were running through, in their own minds, how the robbery would take place, and what the police reaction would be, and all the time they were thinking of ways of disrupting the police activity or giving themselves more time," Ramm said.

"If the car was seen by a bystander leaving the scene, and that person was able to give a description of the car, once the police started making inquiries into that vehicle, those inquiries would become more complicated, more difficult and more time consuming to resolve."

And police say the gang's preparations didn't stop with the getaway car.

A few days before the heist, the bars across the window where the thieves entered the vault were cut, according to authorities. Removing the metal grille completely might have raised the suspicion of passers-by, so the suspects covered their tracks by temporarily sticking the bars back in place with glue, police said.

The window was in a blind spot, so it wasn't visible on security cameras and the whole area was in "complete darkness," the Saxon State Ministry of Culture and Tourism said in response to an inquiry from the Saxon parliament. A motion sensor that should have been triggered by the theft didn't go off. The ministry said the alarm had gone off the day before the crime and security guards failed to reactivate it. CNN reached out to the state prosecutor's office for more details about the alarm failure, but the office wouldn't comment because the investigation is ongoing.

The Heist

At about 4:50 a.m. on Monday, November 25, 2019, the gang sprang into action, according to the police.

First, police said, the thieves or their accomplices set fire to a power distribution box near the Green Vault. This caused the streetlights nearby to go out, plunging the whole area into darkness.

Next, at 4:57, they headed for the vault.

The police said security camera video showed the thieves knew where they were going. After entering the building through the window of the mirrored Hall of Treasures, police believe they hurried through the vault's Heraldry Room straight to the Jewelry Room where the museum's most valuable pieces are on display.

Security camera footage shows it took the robbers just a few minutes to get inside, smash the display case, grab the jewels and leave. The thieves couldn't steal all the pieces in the display, because some were sewn into the cases, Ackermann told German public broadcaster ZDF.

But before they made their escape, the robbers sprayed the room with a powder fire extinguisher to cover their tracks, the police said.

"Footmarks are very often used to identify the footwear used in by criminals," said Ramm. "Fairly often, they'll get rid of gloves and all sorts of other things but forget to get rid of their shoes. So, anything that disrupts the forensic trail is -- I hesitate to say it -- useful."

Police said the robbers escaped the scene in the Audi and that, just 13 minutes after the CCTV camera captured the first images of them entering the vault, the gang's car had been abandoned and set on fire in an underground garage some three miles away. The police connected the car to the robbery almost immediately.

"It is incredibly difficult to use a vehicle and not leave DNA behind," said Ramm. "There have been lots of cases around the world where tiny amounts of DNA have been found and it was enough to tie the person to a car ... so burning the car was all about the DNA."

The police operation, codenamed Operation Epaulette after one of the artifacts stolen that day, began the moment the museum's security staff made their first emergency call -- while the robbers were still inside the building.

The vault's two security guards saw the robbery unfolding on security monitors but did not intervene. That decision was later questioned by the police, but Ackerman said security staff had followed the safety protocols .

Ramm said detectives likely started by looking closely at the museum itself.

"The only way that these things happen is if the robbers have got really good inside information," he explained. "You've got to know that there aren't, for example, laser beams across the room, you've got to know that there aren't pressure sensitive tabs around the place. It is extremely risky to do what they did.

"It is conceivable that they've done extended research on the building," Ramm said.

Saxony's State Prosecutor's office said in March 2020 it was investigating four security personnel from the museum. Last week, the State Prosecutor's office told CNN the investigation is ongoing. A spokesperson said a criminal complaint was lodged against two guards by a private individual, alleging they "did not react adequately and prevent the robbery."

He said two other security guards were investigated. One was suspected of handing documents about the Green Vault and its security systems to the perpetrators and was arrested four days after the heist. The other guard was released following an investigation, he said.

The spokesperson added that a fourth guard was being investigated as "there is evidence of an action in relation to the alarm system, which could have facilitated the theft."

The Investigation

By September 2020, police said they had received hundreds of tip-offs and searched several Berlin properties believed to be connected to the robbery.

They also found out more about the getaway car -- including where it had been resprayed or refoiled -- and released a composite image of one of the suspects.

Then, on November 17, 2020, almost a year after the Green Vault's prized treasures were stolen, the police launched a huge security operation in Berlin, bringing in special forces and 1,638 officers from across Germany.

They were targeting five members of the infamous Remmo Clan, one of Germany's most powerful crime families, which operates mostly in Berlin.

Ralph Ghadban, a political scientist and an expert on clans in Germany, said the way the heist was allegedly carried out and the number of suspects and their possible accomplices involved shows the power the clans wield.

"The clan protects and helps its members, it can have many thousands of members and can dominate and terrorize entire quarters in the city," he said, adding that the "forceful and quick" action displayed during the heist is one of the clan's calling cards.

The police announced the arrest of three of the five prime suspects during the operation in Berlin.

The police identified the two suspects still on the run as twin brothers Abdul Majed R. and Mohamed R.; a massive manhunt was launched to find them.

Interpol issued a red notice for the twins, but it took another month before Mohammed was caught in a car in the Berlin neighborhood of Neukölln -- on the Remmo clan's turf.

Abdul Majed remained at large for another five months before he too was detained on May 17, 2021.

A sixth and final suspect in the case was apprehended in August 2021, police said.

A month later, prosecutors in the case finally charged all six men with crimes including serious gang theft and arson. Three of the suspects are brothers and the other three are their cousins. Two of the accused were previously convicted for stealing a 100-kilo commemorative gold coin known as the "Big Maple Leaf" from Berlin's Bode Museum and are now serving prison sentences for that.

CNN has reached to the representatives of the accused for comment.

The suspects may be in custody, but for the police, the investigation is far from over.

"Something of this nature, where the items themselves are irreplaceable, most of the detectives that I've worked with over the years would think it's a job kind of half done, not to have recovered the items," said Ramm.

So, what happened to those priceless jewels stolen from the display case that day in November 2019?Ramm and other experts believe the likeliest scenario is the one the museum's curators feared the most: that the stolen items have been broken up, the stones sold on, and the precious metals melted down."

All of that takes organization," said Ramm. "It is very rare that the people who actually stole the items will be the people that ultimately disposed of them. There will be a network and that's why the police will be very, very keen to get hold of mobile phones, computers, anything that shows the links between the six people that they're putting on trial shortly and any other criminal groups."

Hard drives, computers and cell phones were indeed seized during the mammoth police investigation but the stolen treasures themselves have vanished without a trace.

The Green Vault remained closed to visitors for months, due to the investigation and later the coronavirus pandemic. When it reopened in May 2020, the burglarized cabinet was repaired but left deliberately empty.

The trial is scheduled to last until at least the end of October. If convicted, the suspects face potential jail sentences of several years.