Twitter, Spotify and Reddit, Paypal, Fox News, and many other websites were down completely or not loading properly Friday morning. Hackers unleashed a large distributed denial of service (DDoS) attack on the servers of Dyn — a major DNS host — reported Gizmodo.com.

“It’s probably safe to assume that the two situations are related,” said the popular tech site.

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A second wave of afternoon attacks seemed to be affecting the U.S. West Coast and also Europe, multiple outlets reported. While it is not clear how the two attacks are related, the outages are reportedly very similar.

“Starting at 11:10 UTC on October 21th Friday 2016 we began monitoring and mitigating a DDoS attack against our Dyn Managed DNS infrastructure,”Dyn posted its website. “Some customers may experience increased DNS query latency and delayed zone propagation during this time. Updates will be posted as information becomes available.”

Gizmodo listed at least 60 popular high-trafficked websites that were offline.

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Domain Name Servers are the Internet’s “phone book,” and facilitate a request to go to a certain webpage, explained Gizmodo. If the DNS provider that handles the request for a certain site is down due to a DDoS attack, the user cannot access the site.

The Department of Homeland and Security and FBI were both investigating Friday’s attack, Reuters reported.