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Online retailer Amazon.com Inc said its U.S. website was down for about 2 hours on Friday but said it was unsure what had blocked access to the world's biggest online store.
An Amazon spokeswoman said the site was unavailable for a couple of hours starting at around 10:25 a.m. California time (1725 GMT). Web monitoring company Keynote put the outage at 2-1/2 hours and said the site was fully back up just before 1 p.m. in California (2000 GMT).

Users in the United States who tried signing onto the popular portal during the outage saw a message saying the service was unavailable. It urged shoppers who were making purchases when it went down to check back later to see if the sales had been properly rung up.

While the cause of the outage remained unclear as of Friday afternoon, one expert said it was likely due to the sheer complexity of the site, which tailors content to visitors depending on factors such as their location, purchase history and behavior during previous visits.

"Amazon is one of the most complex retail sites online today. The more complex something is, the more likely it is to break," said Shawn White, director of operations with Keynote.

He said that less likely scenarios were that the site could have collapsed if it were suddenly overwhelmed by a surge in traffic or succumbed to an attack by hackers.

Those scenarios were less likely, White said, because Amazon has plenty of expertise in dealing with heavy traffic and protecting itself against criminals.

Amazon's international sites, such as amazon.co.jp, amazon.co.uk and amazon.de, were not affected, nor was its online storage service or other Web services that it sells to computer programmers, the company's spokeswoman said.

But Web surfers who tried to access amazon.com, from the United Kingdom and Germany to India, said they were unable to connect with the company's U.S. flagship site.

Internet content delivery company Akamai Technologies Inc, which counts Amazon among its customers, had no problems with its network on Friday, said Akamai spokesman Jeff Young.

Amazon shares fell 4.6 percent to close at $80.63 on the Nasdaq. Akamai shares fell 4.9 percent to $37.38.

(Reporting by Ritsuko Ando, Ken Li in New York; Editing by Phil Berlowitz, Gerald E. McCormick, Gary Hill)http://www.amazon.com