Police said Thursday they had shut down an encrypted phone network used as a key tool by organised crime groups across Europe to plot assassination attempts and major drug deals.

French and Dutch police said they hacked the EncroChat network so they could read millions of messages "over the shoulders" of criminal suspects as they communicated, leading to more than 100 arrests.

EncroChat -- which sells custom encrypted phones -- sent a message to users in June warning them to throw away the devices as its servers had been "seized illegally by government entities". The hack allowed police to "look into the heart" of organised crime groups, Wil van Gemert, Deputy Executive Director of the EU police agency Europol, told a press conference in The Hague.

The hacking of the phones allowed the "disruption of criminal activities including violent attacks, corruption, attempted murders and large-scale drug transports," Europol and the EU judicial agency added in a joint statement. "Certain messages indicated plans to commit imminent violent crimes and triggered immediate action."

French authorities launched the investigation in 2017 after finding that EncroChat phones were "regularly" found in operations against criminal groups and that the company was operating from servers in France. "Eventually, it was possible to put a technical device in place to go beyond the encryption technique and have access to the users' correspondence," Europol and the EU judicial agency Eurojust said in a joint statement.

Between 90 and 100 percent of EncroChat clients were linked to organised crime, according to judicial sources, with around 50,000 of the phones in circulation.

'Power off immediately'

Dutch police then became involved based on information shared by French police. The statement said the investigation "made it possible to intercept, share and analyse millions of messages that were exchanged between criminals to plan serious crimes. "For an important part, these messages were read by law enforcement in real time, over the shoulder of the unsuspecting senders."

Dutch police had busted 19 meth labs, seized thousands of kilos of crystal meth and cocaine, and arrested more than 100 people as a result of the hack, Andy Kraag, head of the police central investigations division, told the press conference. Dutch media said two of the country's most wanted meth smugglers had been arrested as a result of the investigation.

The French and Dutch authorities defended the decision to hack into the encrypted phone network, saying it was justified by evidence that it was mainly being used for criminal ends. "The platform targeted in this operation catered specifically to the needs of criminals," said van Gemert. "The abuse of encryption technology is a key facilitator of criminal activity."

Encrochat, which sold its phones for around 1,000 euros each, sent what it called an "emergency" text to its users on June 13 saying it had been compromised. "Today we had our domain seized by government entities," said the message, a picture of which was included in the statement but has also been shown on news media in recent weeks. "You are advised to power off and physically dispose your device immediately."