While power had been restored to all tenants of Seattle's Fisher Plaza East, its owner Fisher Communications was still investigating the cause of a day-long outage on Monday afternoon, avoiding making a conclusive statement.

"We don't know the cause," Fisher spokesman Ron Low said. "It's still under investigation."

Power was back for all tenants approximately at 2:30 a.m. (PDT) on Saturday, according to Low. It was brought back in phases after a total outage took place around 11 p.m. on Thursday.

Fisher were able to restore power by bringing in temporary generators, as on-site backup generators could not be used because of the damage.

"Fisher brought in electrical generators to power the building while they continue to assess the damage and begin the repair process, Low said.

The plaza houses multiple data centers - including Adhost and Internap - and TV and radio station facilities.

According to the summary and timeline of the incident Adhost released Monday, the outage was caused by two bus ducts burning out in the main switchgear room on the second floor of the building. Switch gear in the room feeds power to the building's three central electrical risers. Equipment in the room is also tasked with automatic switching between primary and generator power sources.

The heat activated the sprinkler system in the room and power was lost, as the switchgear shut down.

After an initial damage assessment was made by Fisher engineers, it became clear that on-site generators and electrical switching equipment could not be restored quickly enough and temporary generators were "rush-ordered."

During the day on Friday, Fisher Plaza engineers and tenants' engineers formulated a plan to "splice temporary generator power onto the risers between the affected areas and the critical loads" and began implementing it.

The first riser was powered shortly after midnight on Saturday morning and the second one was powered one hour later. All three risers were energized by approx. 1:45 a.m. and tenants began gradually powering up their equipment.

Temporary generators had been fueled every eight hours and monitored by Fisher and Cummins engineers.

"...we have limited options in the case that one of the generators fails," Adhost representatives wrote in the summary document.

Among services using data center space at Fisher Plaza East that were affected by the outage were Microsoft's Bing, and CyberSource-owned online transaction service Authorize.net.