SAN FRANCISCO — UltraDNS, a web content delivery service, went down Thursday afternoon, taking with it a number of popular websites, including Netflix and Expedia.

The cause of the 90-minute failure was an internal issue in a server on the East Coast and was not the result of an attack by hackers, said Lara Wyss, an UltraDNS spokeswoman.

Initially, members of the UltraDNS support team indicated the issue stemmed from a DDoS or Denial of Service attack in which hackers flood a service with traffic until it collapses under the load. But after further investigation, the company’s chief information officer said that the issue was not caused by a DDoS assault but by a technical malfunction, Ms. Wyss said in an interview.

The system failure was quickly resolved but had a ripple effect across the Internet, as many companies depend on the company’s cloud service to run their websites. Several frustrated customers, and administrators, broadcast their complaints on Twitter.