The young people who gathered for a party in a Brooklyn apartment building on Sunday night appeared to be in a festive mood, neighbors said. Revelers spilled out of apartments on the third and fourth floors, into hallways and out onto the sidewalk.

But shortly after 11 p.m., the celebration in Bedford-Stuyvesant was overtaken by violence and chaos. At least two people opened fire, leaving one woman dead and six others injured, including a 14-year-old girl, police said.

The episode came amid a surge in violence in New York City this year. Shootings have doubled compared with last year, with at least 1,660 incidents, and murders have risen by about 37 percent, to about 400.

The toll includes several mass shootings: Six people were shot in the Prospect Lefferts Garden neighborhood of Brooklyn in October, and five people, including a 6-year-old boy and his mother, were shot during a Carnival celebration in September.

“We’re seeing a tremendous uptick that has us concerned,” Assistant Chief Judith Harrison, who oversees the police department’s Brooklyn North command, said at an overnight news conference about the shooting.

It was not clear what led to the shooting on Sunday night, though police officials have suggested that it might be connected to a separate, earlier shooting that took place after a “Sweet 16” party in East New York.

A police official on Monday identified the woman who was killed as Daijyonna Long, 20.

A man who left the apartment building on Monday said that Ms. Long had been visiting from Virginia, where she was a college student, to attend the party. The man, who declined to give his name, said Ms. Long was “an innocent bystander.” He carried a bag of what he said were Ms. Long’s clothes.

In a news report from last week about traveling during the pandemic, Ms. Long told a television station in Virginia, WRIC, that she was planning on visiting Brooklyn to attend a relative’s birthday and for Thanksgiving.

“It’s probably just going to be like small relatives, not a big, big party or anything,” she said in the interview. “I feel pretty confident. Yeah, I just wear my mask, stay six feet, use sanitizer all the time, and I guess I’ll be just fine.”

Relatives of Ms. Long could not immediately be reached for comment on Monday afternoon.

The party in Bedford-Stuyvesant, which took place at 15 Albany Avenue, appeared to be a continuation of the earlier “Sweet 16” celebration at a party hall that had been broken up around 7 p.m., the police said.

After that party dispersed on Sunday, a 17-year-old boy who had attended was shot in the leg a block and a half from the hall, the police said. He was expected to survive.

The police believe two gunmen arrived at the Albany Avenue building at around 11 p.m. and opened fire outside the front entrance. They then walked into the lobby and continued firing as they walked upstairs to the third floor, the police said.

The victims included a 16-year-old boy, who was hit in the torso and was in critical condition at Kings County Hospital Center, and a 15-year-old boy, who was shot in the leg and was in stable condition at Maimonides Medical Center, the police said.

The 14-year-old girl was struck in the buttocks and was rushed to Maimonides, where she was in stable condition.

Three others were wounded but were expected to survive: an 18-year-old woman shot in the leg, an 18-year-old man also struck in the leg and a 19-year-old man hit in the torso, the police said.

Deputy Chief Michael Kemper, who oversees the detective bureau in Brooklyn North, said during a news conference that the shooting in Bedford-Stuyvesant could be a “retaliatory shooting” or a continuation of the shooting in East New York.

Borough President Eric Adams of Brooklyn said that the Bedford-Stuyvesant shooting stemmed from a dispute that began at the East New York party hall.

“A beef that started in East New York spilled over here,” he said.

One resident of 15 Albany Avenue, Fred Waller, said he was returning to his apartment on the second floor at around 11 p.m., when he saw two people arguing in front of the building among a larger group of about 75 people.

Mr. Waller said he also saw people partying inside the building. After he entered his apartment, he said he heard about a dozen gunshots ring out. He stayed hidden inside his apartment for a few minutes before looking out and spotting a young man with gunshot wounds slumped on the stairs as partygoers streamed out of the building.

“This is out of control,” Mr. Waller said of the violence outside his door.

A woman who was leaving the building on Monday morning said she was at a party on the fourth floor when a skirmish among a separate group of partygoers erupted on the third floor. She said she heard five shots.

“I don’t know who was shooting,” said the woman, who did not want to be identified for safety reasons.

James Wilson, who has lived across the street for a decade, said he came home around 10:30 p.m. and saw a crowd of around 30 young people in front of 15 Albany.

The people he saw appeared to be happy and having fun, Mr. Wilson said. He added, however, that the building has previously been the site of disruptive parties.

Ashley Southall contributed reporting and Kitty Bennett contributed research.