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.

UltraDNS is considered one of the top providers in the web content delivery space. It and competitors like Akamai and CloudFlare play a key role in directing and managing web traffic for businesses that depend on the service for website speeds and protection against DDoS attacks. The company was acquired in 2006 by Neustar, an information services company based in Sterling, Va.

The failure on Thursday was the second time in two years the service has gone down, according to Dotcom Monitor, a company in Minneapolis that monitors web speeds and performance. In January 2013, UltraDNS suffered a similar failure that affected customers around the world.

The assumption that UltraDNS was under attack was not unreasonable given an increase — over the past 18 months — in high-volume DDoS attacks that exploit the Internet’s switchboard system, called the Domain Name System, or DNS, to amplify the attacks. Many of those attacks have come from China against China-specific websites, said Matthew Prince, the chief executive at CloudFlare.

Attackers are now sending forged traffic requests that appear legitimate, making it difficult to root them out. These attacks come at such a high rate — in some cases 500 million requests per second — that only the toughest websites can fend them off, Mr. Prince said.