Days after a service outage temporarily left BlackBerry users in North America without access to their email, Research In Motion said an initial investigation indicated that the outage was caused by problems with an internal data routing system that recently had been upgraded.

The upgrade was part of an ongoing effort to increase network capacity, "but there appears to have been a problem with this specific upgrade that caused the intermittent service delays," RIM said in a statement sent via email late Tuesday. No further explanation was provided.

RIM repeated earlier comments that the BlackBerry service was restored quickly and that no messages were lost during the outage, which started at about 3:30 pm EST on Monday.

"RIM continues to focus on providing industry-leading reliability in its products and services, and continues to invest in its infrastructure and processes," the company added in its statement. It concluded by apologising to customers for any inconveniences that they experienced as a result of the problems.

Tuesday's outage was the second in less than a year that RIM blamed on an upgrade to the systems that support the BlackBerry service, which now has about 12 million subscribers.

Last April, the Waterloo, Ontario-based company said the flawed installation of cache optimisation software led to a half-day outage, which was worsened by a failure to switch the service over to a backup system. At the time, RIM promised that it would bolster some of its testing, monitoring and recovery processes in an effort to prevent repeat episodes.

It wasn't clear how many users were affected by the latest snafu, although a Verizon Wireless spokesman said that only data transmissions were affected - not phone calls. Email access appeared to be hampered for customers of all US wireless carriers, including Verizon Wireless, AT&T and Sprint Nextel, according to users and analysts.

Phillip Redman, an analyst at Gartner, said it will be "critical to keep the highest service levels" possible for the service, especially as the number of BlackBerry users grows even further. If outages at RIM become a trend, they "will push users toward other options that don't have a single point of failure," Redman said.