Power outage darkened Detroit

• NEW: Detroit mayor says he hopes power will be restored by end of day


• Power goes out at public buildings


• People trapped in elevators in several government buildings


• More than 80 Detroit public schools dismiss students early

(CNN) -- Detroit Mayor Mike Duggan said a power outage put city schools, firehouses, police stations, City Hall and Wayne State University in the dark Tuesday morning.

Speaking at a press conference, Duggan said power had been restored to about one third of the customers by 2 p.m. ET and that power probably will be restored to all by the end of the day.

The outage affected about 900 locations, including 36 fire stations and 81 city schools, according to the Detroit website.

Public schools dismissed students after the power went out. Duggan said the dismissal went smoothly.

The mayor said there had been no spike in police or fire calls while power was out. About 18 state troopers helped 78 officers direct traffic at intersections while traffic signals were out of service, he said. Schools and other public buildings should be back in service Wednesday, officials said.

The city website said 740 traffic signals were knocked out, but power was not lost for the 911 system.

The outage occurred about 10:40 a.m. ET when a Public Lighting Department (PLD) power cable and then a circuit breaker failed, he said.

The City of Detroit is going through bankruptcy. As part of the process, the city is getting out of the business of running PLD and the utility corporation DTE Energy is taking over, Duggan said. DTE Energy assisted the city in bringing the system back online.

"All we know at this point is that the cable went to ground, which means it grounded itself out," said Jerry Norcia, president and chief operating officer for DTE Electric and Gas Storage & Pipelines. "So it could be a breach in the insulation or it could be many other circumstances that caused that cable to fail."

Detroit has experienced notable power outages in the past.

Just last Sunday, power went out during a professional hockey game at Joe Louis Arena. Detroit's Comerica Park lost power last year during the American League Championship Series.

The outage trapped some people in elevators in several government buildings, and firefighters were trying to free them, fire Chief Dale Bradley said.

This outage isn't believed to have affected residences, officials said.

Duggan said he'd earlier asked DTE to focus on those circuits that fail the most and review them.

The cable that failed Tuesday was not one that had done so previously, authorities said.

The power outage won't affect Tuesday night's professional hockey game, said Todd Beam, Red Wings director of public relations.

"We are back at full power at Joe Louis Arena," he said. "The game will be played as scheduled at 7:30 p.m. tonight."

CNN's Kevin Dotson contributed to this article.