North Korean hackers stole nearly $400 million worth of cryptocurrency in 2021, making it one of the most lucrative years to date for cybercriminals in the severely isolated country, according to a new report.

Hackers launched at least seven different attacks last year, primarily targeting investment firms and centralized exchanges with a variety of tactics, including phishing, malware, and social engineering, according to the report from Chainalysis, a firm that tracks cryptocurrency.

The cybercriminals worked to gain access to organizations’ “hot” wallets — digital wallets that are connected to the internet — and then move funds into DPRK-controlled accounts.

A confidential United Nations report previously accused the regime of North Korea’s leader, Kim Jong Un, of conducting “operations against financial institutions and virtual currency exchange houses” to pay for weapons and keep North Korea’s economy afloat.

Last February, the US Justice Department charged three North Koreans for conspiring to steal more than $1.3 billion from banks and companies around the world and orchestrating digital heists of cryptocurrency.