[What you need to know to start the day: Get New York Today in your inbox.]

Throughout the weekend, officials at Con Edison had assured its customers that the power grid would hold. Then, around 7 p.m. on Sunday, as demand for electricity rose, the utility’s system was stressed as it had rarely been on a weekend.

The load on the grid was setting a weekend record: 12,063 megawatts.

The biggest trouble, officials said, came in southeast Brooklyn, in and near the neighborhoods of Canarsie, Mill Basin, Flatlands and Bergen Beach.

Three of the 19 cables that snake out of a substation to deliver electricity to homes and businesses in the area had failed. Then, in a span of about 10 minutes, a fourth and then a fifth failed, Tim Cawley, the president of Con Edison, recounted in an interview.

That left 14 cables to handle a heavier-than-normal electricity flow, leaving the system’s operators worried that the entire network in the area could fail, Mr. Cawley said.