The outage, discovered by San Mateo, Calif.-based Keynote's Internet performance monitoring systems, occurred Monday between 11:02 a.m. and 12:42 p.m. PDT, said Keynote spokesman Dan Berkowitz.

"Most users around the world attempting to access the Amazon.com Web site would not have been able to access the site" during the outage, Berkowitz said in an e-mail message. "Keynote considers this to be a fairly significant outage."

In an interview Wednesday, Berkowitz said Keynote does not know the exact cause of the outage, but he pointed to a back-end system problem as one possibility.

Amazon.com Inc. spokesman Craig Berman said Wednesday that he couldn't discuss the cause of the outage because the company doesn't comment on its back-end systems.

"We strive every day to provide the best experience for our customers, and we have a Web site that is very complex," he said. If a problem arises, Amazon works "to get it fully functional as quickly as possible, which is what we did" after Monday's incident, he added.