Research In Motion Ltd. said it has fully restored service to its global network early Thursday, as it ramped up communication with carriers and customers—apologizing for the continued delays.

Co-chief executive Mike Lazaridis—who has stayed out of the public eye since Monday, when outages first struck Europe and other markets—appeared on a conference call early Thursday. He apologized for the outages and said the backlog of messages that had clogged systems since Monday is being cleared up.

The outage started with a failure in RIM's European data center, and cascaded across the world. Mr. Lazaridis said the outage was the largest in its history and likely caused by a hardware problem. "We don't know why the switch failed in the particular way that it did," he said.