One of the world’s biggest networks of hijacked computers, which is suspected of attacking online banking customers, has been targeted by police swoops in 10 countries, German police said on Thursday

In an internationally coordinated campaign, authorities carried out the raids on Wednesday, seized servers and website domains and arrested suspected leaders of a criminal organization, said police and prosecutors in northern Germany.

Officials said they had seized 39 servers and several hundred thousand domains, depriving criminals of control of more than 50,000 computers in Germany alone. These hijacked computers were used to form a “botnet” to knock out other websites.

The strike came in the same week that hackers tried to create the world’s biggest botnet, or an army of zombie computers, by infecting the routers of 900,000 Deutsche Telekom with malicious software.

The attack failed but froze the routers, causing outages in homes, businesses and government offices across Germany on Sunday and Monday, Deutsche Telekom executives said.

Police said criminals had used the botnet targeted in Wednesday’s international raids and known as “AVALANCHE” since 2009 to send phishing and spam emails. More than a million emails were sent per week with malicious attachments or links.

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When users opened the attachment or clicked on the link, their infected computers became part of the botnet.

The raids came after more than four years of intensive investigation by specialists in 41 countries.

Authorities have identified 16 suspected leaders of the organization from 10 different countries.

A court in Verden, northern Germany, has issued arrest warrants for seven people on suspicion of forming a criminal organization, commercial computer fraud, and other criminal offenses.