The attackers apparently used tens of thousands of hacked internet of things devices—household appliances such as digital video recorders, security cameras, and internet routers—to generate a massive amount of digital traffic. That digital noise was sent to Dyn, a domain name service provider used by major online companies, disrupting its ability to translate human-readable internet addresses into the IP addresses networks use to route traffic.

The attack came after years of warnings from security experts that the makers of many internet-enabled devices paid too little attention to security, shipping internet-connected hardware with preset passwords, insecure default connections, and other vulnerabilities.

“It is just a matter of time until attackers find a way to profit from attacking IoT devices,” a report from security firm Symantec warned last year. “This may lead to connected toasters that mine cryptocurrencies or smart TVs that are held ransom by malware. Unfortunately, the current state of IoT security does not make it difficult for attackers to compromise these devices once they see the benefit of doing so.”

Hackers and security researchers have previously exploited vulnerabilities to get access to devices like baby monitors and webcams. Researchers from security company Pen Test Partners even demonstrated earlier this year how hackers could install ransomware on an internet-connected thermostat, leaving victims sweltering or shivering until a ransom is paid.

And in Friday’s attack, compromised IoT devices were coordinated as part of a botnet—a network of hacked machines essentially turned into remote-controlled robots by malware—dubbed Mirai. Between 500,000 and 550,000 hacked devices around the world are now part of the Mirai botnet, and about 10% of those were involved in Friday’s attack, said Level 3 Communications chief security officer Dale Drew on the internet backbone provider’s Periscope channel Friday.

“With a rapidly increasing market for these devices and little attention being paid to security, the threat from these botnets is growing,” according to a blog post published by Level 3 just days before the attack.