On Monday, authorities busted an international archaeological crime scheme in a sting dubbed “Operation Achei.” Per a press release, more than 350 police officers across five countries worked together to recover 10,000 ancient Greek and Roman artifacts stolen from archaeological sites in the Calabria region of southern Italy.

The Italian Carabinieri Department for the Protection of Cultural Heritage led the investigation with support from the European Union’s law enforcement agency, Europol. The Carabinieri’s “culture commandos” have the skills of “archaeologists, paleontologists, art historians and combat-trained shock troops,” wrote National Geographic’s Frank Viviano in 2015.

Operation Achei began in 2017 with a focus on Calabria, the “toe” of Italy’s boot-shaped peninsula. The artifact traffickers allegedly used a backhoe-like excavator to dig up sections of known archaeological sites near the Hera Lacinia, according to the Associated Press. They then sifted through the disturbed areas with sophisticated metal detectors, Italian police officials told the Guardian’s Lorenzo Tondo. The illicit excavators wore ski masks to hide their identities, but during one heist, the license plate of a parked car showed up on the police’s drone video surveillance.

After collecting the artifacts, the group conveyed the items to people who could carry them abroad, “where they were put up for auction in important international auction houses and sold at very high figures,” the investigators said at a press conference reported by the Guardian.

Police from France, Britain, Germany and Serbia assisted Italian authorities with the operation. Eighty home searches yielded artifacts from as early as the 4th century B.C. The recovered items include ancient jars, plates and jewelry worth millions of euros.

“The damage caused to the Italian cultural heritage by this criminal group is very significant as … the criminals were looting archaeological sites for many years,” Europol says in the statement.

Two alleged leaders of the illegal archaeology scheme have been jailed, and 21 other suspects remain under house arrest in Italy.

Illegitimate archaeological digs are regular occurrences in Italy, but the Carabinieri are specifically trained to catch perpetrators. Officers must study art history, archaeology and international legal conventions at the University of Rome, as well as “demonstrate exceptional investigative skills,” Captain Lanfranco Disibio, leader of the squad for Tuscany and Umbria, told National Geographic’s Viviano in 2015. In 2014 alone, Viviano notes, the officers recovered around 130,000 artifacts worth more than $500 million.

There is still plenty of work left to do: As the Guardian reports, more than one million Italian artifacts remain missing today.