New York City ? The company behind the BlackBerry smart phones says Monday’s three-hour outage was caused by an upgrade designed to increase capacity.

Research in Motion says the upgrade was part of “routine and ongoing efforts,” and that similar upgrades in the past resulted in no problems.

The outage, which started about 2:30 p.m., annoyed subscribers who are used to having their e-mails available whenever they’re in cellular coverage. It affected only some of the BlackBerry users in North America. For others, the service kept working.

It was the second major outage for the service in less than a year.