A number of popular websites like Twitter and Netflix were down for some users on Friday in what appears to be a massive cyber attack, which was being investigated by the U.S. Department of Homeland Security.

Dyn, which manages website domains and routes internet traffic, experienced two distributed denial of service attacks on its DNS servers. A DDoS attack is an attempt to flood a website with so much traffic that it impairs normal service.

Affected sites included Twitter, Etsy, Github, Vox, Spotify, Airbnb, Netflix and Reddit.

“If you take out one of these DNS service providers, you can disrupt a large number of popular online services, which is exactly what we’re seeing today,” said Jeremiah Grossman, chief of security strategy at cybersecurity startup SentinelOne.

“Starting at [7:00 a.m. ET] on October 21… we began monitoring and mitigating a DDoS attack against our Dyn Managed DNS infrastructure,” the company said in a statement.

By 9:20 a.m., Dyn said “the services have been restored to normal.”

But less than three hours later, Dyn said it was again monitoring an attack against its DNS server.

Reuters reported that the FBI and Department of Homeland Security were investigating.

Initially, outages were primarily impacting those on the East Coast, but by midday Friday, people in Europe were reporting outages as well.

No one has claimed responsibility for the attack yet. A government official said the U.S. is “looking at all possible scenarios including possible cyber activity.”

While little information is available about the cause or who is behind the attack, companies turned to Twitter to tell customers to stay patient.

“Uh oh, we’re having some issues right now and investigating. We’ll keep you updated!” tweeted Spotify.

“A global event is affecting an upstream DNS provider. GitHub services may be intermittently available at this time,” tweeted GitHub.