A gas leak forced the temporary evacuation Friday of hundreds of residents in Lawrence, a city still reeling from a destructive series of gas explosions and fires a year ago.

Residents in about 150 homes near South Broadway and Salem Street evacuated in the early morning hours after a police officer smelled gas around 3:15 a.m. and alerted authorities, officials said.

The leak was located and sealed off by 5 a.m., Columbia Gas of Massachusetts said. There were no reported injuries, explosions or fires.

Gov. Charlie Baker and others declined to say what caused the leak, but Baker said investigators are confident that the leak was isolated and that there's no threat to public safety.

The state Department of Public Utilities will investigate in the days ahead, he said.

By Friday afternoon, electricity and gas were gradually being restored, and the majority of residents were allowed to return to their homes.

Lawrence Mayor Daniel Rivera said the city would continue to operate a temporary shelter for the remaining residents.

Another, smaller gas leak was also reported Friday afternoon. It was quickly resolved and not related to the larger morning episode, Rivera said.

"It's a strange thing to be back talking about gas only a year after this thing happened," he remarked at one point.

Columbia Gas, which has been blamed for last year's disaster , said its crews were been working in the area at the time.

The line was among miles of new gas pipelines the company has installed across the region to replace old cast iron pipes, it said.

"We know that it was not systemic across the area," said Mark Kempic, the company's president. "That's why people should feel safe."

The National Transportation Safety Board concluded this week that Columbia Gas poorly planned a routine pipeline replacement project in Lawrence, causing natural gas overpressurization that led to the explosions and fires in homes and businesses on Sept. 13, 2018.

One person died, dozens of others were injured, about 100 structures were damaged, and thousands of residents and businesses were a left without heat or hot water, for months in some cases.