(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.  —read more—