As part of an ongoing initiative targeting computer attack “booter” services, the U.S. Justice Department announced the court-authorized seizure on May 8 of 13 internet domains associated with DDoS-for-hire services.

The seizures last week are the third wave of U.S. law enforcement actions against booter services that allowed paying users to launch powerful cyberattacks that flood targeted computers with information and prevent them from being able to access the internet.

Data shows that hundreds of thousands of registered users have used the services to launch millions of attacks against millions of victims. School districts, universities, financial institutions and government websites are among the victims.

Ten of the 13 domains seized are reincarnations of services that were seized during a prior sweep in December, which targeted 48 top booter services. For example, one of the domains seized – cyberstress.org – appears to be the same service operated under the domain cyberstress.us, which was seized in December. Authorities said investigations into booter services remain ongoing.

With the domains seized on May 8, the FBI opened or renewed accounts with each booter service and used cryptocurrency to pay for subscription plans. Each service was tested by using the website to launch DDoS attacks on computers controlled by the FBI. The FBI then observed the effects of the attacks at their “victim” computers, confirming that the booter websites operated as advertised. In some cases, despite the “victim” computer being on a network with a large amount of capacity, the test attack was so powerful that it completely severed the internet connection.