"The equipment did not function properly," Rice said. "At this particular time, based upon our analysis, we cannot say definitively that there was a defect in design. What we do know is that the equipment for some unknown reason, at this particular time, did not react the way that it should have."
Asked if Entergy and SMG still plan to hire a third-party investigator to get to the bottom of the cause, Rice said that possibility remains open.
"We'll work closely with SMG, and if there is a need for a third-party investigation, we will do that," Rice said, adding that Entergy was also working with the relay manufacturer.
Shabab Mehraeen, an assistant professor of electrical engineering at Louisiana State University, said relays are common electrical fixtures in businesses and massive facilities such as the Superdome.
"They are designed to keep a problem they sense from becoming something bigger, like a fire or catastrophic event," he said.
The devices vary in size. Mehraeen, who was not familiar with the relay at the Superdome, said he "wouldn't be surprised if it was bigger than a truck."
The reasons the devices fail are the subject of much academic research into the interaction of relays with the complex electrical systems they regulate.
"It's not unusual for them to have problems," Mehraeen said. "They can be unpredictable, despite national testing standards recommended by manufacturers."
Entergy and SMG had both upgraded lines and equipment in the months leading up to the Super Bowl. Rice said the new gear, with the faulty relay, was installed as part of a $4.2 million upgrade by Entergy that included a new power line dedicated solely to the stadium.
In a separate project, SMG replaced lines coming into the stadium after managers expressed concerns the Superdome might be vulnerable to a power failure like the one that struck Candlestick Park during an NFL game in 2011.
Thornton stressed Friday that the dome was drawing only about two-thirds of its power capacity Super Bowl night. He said typical NFL games in late August or September can draw a little more.
Friday's announcement appeared to absolve Superdome officials of any missteps in the blackout.
City officials had worried that the Super Bowl outage might harm New Orleans' chances of getting another NFL championship game.
But NFL Commissioner Roger Goodell downplayed that possibility, saying the league planned to keep New Orleans in its Super Bowl plans. Mayor Mitch Landrieu said the city intends to bid for the game again in 2018.
Associated Press Writer Michael Kunzelman in New Orleans contributed to this report.