With temperatures over 90 degrees Fahrenheit by 8 a.m., about 100 firefighters fought the heat and the flames Sunday morning as a blaze tore through a house in Richmond Hill in Queens, killing a mother and her 7-year-old daughter, and critically injuring the woman’s two teenage sons. Fire Department officials said they were investigating whether the fire was linked to the family’s air conditioner, which neighbors said was located on the first floor.

Police said Silvia Umana, 51, lived in the home with her daughter, Lupe Perez, and two sons, whom neighbors identified as Gilbert, 19, and Gabriel, 15. The younger boy escaped out a second-story window and was rescued by firefighters.

“They always complained about how hot their house was,” said Tiffany Elahie, 14, a friend of the children who lived a few doors away.

It was a tragic coda to a sweltering heat wave that began Friday, with temperatures that hovered consistently in the mid-90s. In some neighborhoods, the heat index was more than a dozen degrees higher.

New York fire officials said they had expanded emergency service crews in anticipation of a surge in calls, and since Friday had responded to more than 230 heat-related incidents, the majority involving older patients.

Across the five boroughs, many New Yorkers stayed inside — and cranked up air conditioning. But by Sunday night, Consolidated Edison was reporting more than 19,500 customers were without electricity; most of the power failures were in the Brooklyn neighborhoods of Gravesend and Park Slope, and Whitestone in Queens.

“It is the third day of the heat wave, so the system is really baking at this point,” said Alfonso Quiroz, a spokesman for Con Edison.

According to its power failure map, Con Edison customers in Gravesend were expected to have power returned by 10 p.m. Sunday. But in Park Slope and Queens, the company estimated residents could be without power until Monday afternoon.

PSEG Long Island had wrestled with its own power failure Saturday night that left thousands without power in the Rockaways.

By Sunday night, both companies were asking customers across neighborhoods in Brooklyn and Queens to limit the use of electrical appliances. Con Edison said it had reduced voltage by 8% in affected areas to protect equipment and maintain service as repairs were made.

“Because the heat is still so heavy today, if customers could alleviate some of the stress to the system it would help overall,” said Elizabeth Flagler, a PSEG Long Island spokeswoman.

Quiroz said Con Edison had hit “record high-power demand” for a weekend.

New York, like much of the country, has been struggling under intense heat, pushing officials in the city to declare a state of emergency. Meteorologists have issued extreme heat advisories stretching from the East Coast through the panhandle of Texas and the Midwest.

Halfway up that Northeast corridor, the heat delayed a Greyhound bus traveling Sunday morning to New York from Boston. Matt Joyal, a passenger, said that customers were stuck waiting on the side of the highway for a replacement bus, which he said they were told would take two to three hours to arrive.

Showers and thunderstorms, and with them cooler temperatures, were expected toward the Midwest. In New York, rain — and relief from the heat — was expected Monday.

A handful of New Yorkers did emerge on Sunday morning to take advantage of the relatively cooler morning temperatures, with handball and slow-pitch softball games carrying on in Brooklyn. Wilfredo Perez, 29, ran ice cubes across his hands in between games at handball courts in Coney Island. “The heat doesn’t bother me,” Perez said. “It actually makes the ball bounce better.”

As the heat wave stretched into a third day, even law enforcement agencies were losing patience. The New York City Police Department said on Twitter that “Sunday has been canceled,” and in Braintree, Massachusetts, police requested that anyone thinking of committing a crime wait until Monday, when it cooled down.

“We are asking anyone thinking of doing criminal activity to hold off until Monday. It is straight up hot as soccer balls out there,” the Braintree Police Department said in a now-viral Facebook post.

New York’s Department of Correction said it had received at least 13 heat-related complaints over the weekend. Latima Johnson, a spokeswoman for the department, was unable to say how high temperatures had reached inside the city’s jail facilities.

The 311 system, however, had received 162 heat-related complaints about the city’s jails, including 94 on Saturday. William Reda, a 311 spokesman, said the system does not track whether calls come from an inmate or a relative.

As part of its response to extreme temperatures, the city opened hundreds of cooling centers this weekend, including one at the Jacob A. Riis Settlement House, a community center serving Queensbridge Houses residents in Long Island City. It did not draw a huge crowd, but the people who took advantage of it, like the group assembled for a tenants association meeting, were grateful.

One bodega owner lamented his struggle to keep ice in stock, and said he had not figured out how to keep his water bottles cool.

“We stocked up on ice again last night, and after two hours today, we ran out,” said Kenny Cheng, 40, who runs a corner shop called James Market in Manhattan’s Chinatown. “People bought it all up.”

Glimpses of a more innocent New York surfaced as residents without air conditioning poured into the streets. They queued up in long lines for community swimming pools, played in the water shooting from fire hydrants and sought shade wherever they could find it.

Dream Harris was among the young opportunists.

“Ice-cold water! One dollar!” Dream, 7, shouted Saturday afternoon from the corner of 152nd Street and Morningside Avenue in Harlem. Supervised by her mother, she had made $30 in an hour.

She planned to keep working until she was out of water. At that point, the plan was to dash through the sprinklers in St. Nicholas Park, pick up some more bottles and get back to work.

This article originally appeared in The New York Times.