Cyber attacks targeting the internet infrastructure provider Dyn disrupted service on major sites such as Twitter and Spotify, mainly affecting users on the US= East Coast.
It was not immediately clear who was responsible. Officials told Reuters that the US Department of Homeland Security and the Federal Bureau of Investigation were both investigating.
The disruptions come at a time of unprecedented fears about the cyber threat in the United States, where hackers have breached political organisations and election agencies.
Homeland Security last week issued a warning about a powerful new approach for blocking access to websites - hackers infecting routers, printers, smart TVs and other connected devices with malware that turns them into "bot" armies that overwhelm website servers in distributed denial of service attacks.
Dyn said it had resolved one attack, which disrupted operations for about two hours, but disclosed a second attack a few hours later that was causing further disruptions.
In addition to the social network Twitter and music-streamer Spotify, the discussion site Reddit, hospitality booking service Airbnb and The Verge news site were among companies whose services were disrupted on Friday.
Amazon.com Inc's web services division, one of the world's biggest cloud computing companies, also reported a related outage, which it said was resolved early Friday afternoon.
Dyn is a Manchester, New Hampshire-based provider of services for managing domain name servers (DNS), which act as switchboards connecting internet traffic. Requests to access sites are transmitted through DNS servers that direct them to computers that host websites.
Its customers include some of the world's biggest corporations and Internet firms, such as Pfizer, Visa, Netflix and Twitter, SoundCloud and BT.
Dyn said it was still trying to determine how the attack led to the outage but that its first priority was restoring service.
Attacking a large DNS provider can create massive disruptions because such firms are responsible for forwarding large volumes of internet traffic.