CNN  — 

It was supposed to be the underworld’s impenetrable communication tool, a digital safe space to plot crimes ranging from drug trafficking to murder away from the prying eyes of the law.

But for nearly three years, an encrypted app used by criminals was covertly monitored by the FBI and Australia’s Federal Police (AFP) – leading to hundreds of arrests and tens of millions of dollars in asset seizures, authorities revealed on Tuesday.

In a statement, the AFP said they and the FBI had been reading the clandestine communications of criminals since 2018 on the ANoM app – a black-market product only accessible on specially prepared mobile phones.

According to the AFP, intelligence gathered from the decrypted messages led to the arrest of 224 suspects on more than 500 charges and the seizure of 3.7 metric tons of drugs and nearly $35 million in cash over the past three years in Australia.

A number of those charged are allegedly linked to Australian-based Italian mafia, outlaw motorcycle gangs, Asian crime syndicates and Albanian organized crime, the AFP added.

“Operation Ironside,” as the investigation was named, began three years ago as a collaboration between several global law enforcement agencies, including the FBI, the AFP, New Zealand police and Europol.

Handout images from the Australian Federal Police show arrests and seizures made under Operation Ironside.

Speaking at a news conference on Tuesday, AFP Commissioner Reece Kershaw said multiple criminal plots were also foiled thanks to law enforcement access to the app, including a planned mass shooting at an Australian suburban cafe and the assassination of a family of five.

ANoM could only be found on phones bought through the black market, which had been stripped of the capability to make calls or send emails, according to the AFP. The phones could only send messages to another device that had the app and criminals needed to know another criminal to get a device.

“The devices organically circulated and grew in popularity among criminals, who were confident of the legitimacy of the app because high-profile organized crime figures vouched for its integrity,” the AFP statement said.

Unknown to the app’s users, the FBI had access to the app and its communications, which the organization then used to collect information on criminal operations.

According to court documents released by the US government on Tuesday, the ANoM was given to San Diego FBI agents by a developer who had created it to compete on the market for encrypted communications platforms.

The developer, whose identity is not revealed, offered it to help the FBI with its ongoing investigations, in return for a reduced sentence on charges he was facing.

In May 2021, the top five countries where ANoM devices were used were Germany, the Netherlands, Spain, Australia, and Serbia, according to the documents.

Kershaw said the criminals didn’t use codes or pseudonyms on ANoM, brazenly discussing crimes in the mistaken confidence their communications were safe.

The Australian Federal Police and other international law enforcement agencies seized tonnes of drugs and millions of dollars in cash as part of Operation Ironside.

“Essentially, we have been in the back pockets of organized crime and operationalized a criminal takedown like we have never seen,” Kershaw said.

“The use of encrypted communication apps presents significant challenges to law enforcement and ANoM has given law enforcement a window into the level of criminality that we have never seen before on this scale.”

Hundreds of arrests and tons of drugs were also seized overseas as a result of the operation, the AFP said, but didn’t specify where. Police also seized assets in Australia which authorities estimated would be worth millions of dollars once they were sold.

In total over the course of the three years, more than 9,000 police officers across 18 countries were involved in maintaining the operation.

The AFP said more arrests are expected in Australia and overseas as a result of the operation.

Kershaw added there were other, larger encrypted communication apps which police were working to access.

CNN’s Angus Watson and Sarah Faidell contributed to this article.