The U.S. Department of Homeland Security is investigating "all possible causes" of an internet outage that affected several popular websites Friday, a spokeswoman for the department said. The outages began in the eastern United States and spread across the country, and even to parts of Western Europe. WATCH: White House spokesman Josh Earnest on what they know Sites affected included the social network Twitter, money transfer services PayPal, music-streamer Spotify and the discussion site Reddit. Internet users affected by the outage 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 cyberattack early Friday, only to face a second attack later in the day. White House spokesman Josh Earnest called the attack "malicious" and said the Department of Homeland Security is monitoring the situation. He added that he had no information about who could be behind the attack. 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.