PARIS, France (AFP)—French authorities have detained seven people in an investigation into the kidnapping of a young Swiss citizen believed to be the latest abduction targeting the hugely lucrative cryptocurrency world, a source close to the case said on Thursday.

The man, aged 20 according to Swiss media reports, was freed on Sunday in the southeastern city of Valence in a special operation by French gendarmes, said the source, asking not to be named.

No details were given regarding how long he had been held. Some 150 gendarmes were mobilised in the operation to free him.

According to the Dauphine Libere regional daily, the young man was found tied up in a house near the Valence high speed train station.

French authorities have been dealing with a string of kidnappings and extortion attempts targeting the families of high-worth individuals dealing in cryptocurrencies.

In January kidnappers seized French crypto boss David Balland and his partner. Balland co-founded the crypto firm Ledger, valued at the time at more than $1 billion.

Balland’s kidnappers cut off his finger and demanded a hefty ransom. He was freed the next day, and his girlfriend was found tied up in the boot of a car outside Paris.

In May, the father of a man who ran a Malta-based cryptocurrency company was kidnapped by four hooded men in Paris. The victim, whose finger was severed by the kidnappers and for whom a ransom of several million euros was demanded, was released 58 hours later during a raid by the security forces.