Cyber attacks targeting a little known internet infrastructure company, Dyn, disrupted access to dozens of websites today, preventing some users from accessing PayPal, Twitter and Spotify.
It was not immediately clear who was responsible for the outages that began in the Eastern United States, and then spread to other parts of the country and western Europe.
The outages were intermittent, making it difficult to identify all the victims. But technology news site Gizmodo named some five dozen sites that were affected by the attack.
They included CNN, HBO Now, Mashable, the New York Times, People.com, the Wall Street Journal and Yelp.
US officials said that the US Department of Homeland Security and the FBI were 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 morning attack, which disrupted operations for about two hours, but disclosed a second attack a few hours later that was causing further disruptions.
Dyn initially said the outage was limited to the Eastern United States. Amazon later reported that the issue was affecting users in western Europe.
PayPal Holdings Inc said that the outage prevented some customers in "certain regions" from making payments.
It apologised to customers for the inconvenience and said that its networks had not been hacked.
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 today.
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 this afternoon.
Dyn is a UK, US-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.
Dyn said it was still trying to determine how the attack led to the outage but that its first priority was restoring service.