The U.S. Department of Homeland Security is investigating "all possible causes" of an internet outage which affected several popular websites Friday, a spokeswoman for the department said. WATCH: White House spokesman Josh Earnest on what they know so far Internet users on the U.S. east coast experienced sluggish surfing as a result of a distributed denial of service (DDOS) attack, according to service provider Dyn. The company said that it had resolved one cyber attack, but its engineers are continuing to work on fixing a second attack. “This morning, October 21, Dyn received a global DDoS attack on our Managed DNS infrastructure in the east coast of the United States,” according to a statement from Dyn, an Internet performance management company. “DNS traffic resolved from east coast name server locations are experiencing a service interruption during this time. Updates will be posted as information becomes available.” Sites affected included Twitter, SoundCloud, Spotify, Airbnb and Reddit. Users in Europe and Asia were not affected by the problem. In a DDOS attack computers flood a targeted system sucking up all the bandwidth, leaving regular users unable to connect or to connect at significantly slower speeds. The attacks are usually caused by compromised systems. According to Dyn, the issues started at 11:10 UTC Friday.