An 18-year-old man was shot and killed during a party at a Brooklyn apartment building early Sunday. The gunman apparently fired a single bullet into a crowd, police said.

Detectives believe Tyquell Walker was the unintended victim of the shooting that happened at about 1:15 a.m. inside the Surfside Houses, a public-housing development on West 33rd Street in Coney Island, a law-enforcement official with knowledge of the case said.

The official said Mr. Walker was at the party in a 12th-floor apartment when an unidentified man took out a handgun and waved it at other guests and stated "get back."

The suspect began backing out of the apartment and was in an elevator when he fired off a shot into a group of people standing in the hallway, the official said. Mr. Walker was struck once in the shoulder and the bullet entered his chest cavity, the official said.

Responding officers discovered the victim outside the building, police said, and he was taken by ambulance to Coney Island Hospital, where he died. The gunman fled and has not been found.

About 15 minutes earlier, a 19-year-old man was shot and killed in a separate incident on East 48th Street in the Flatlands sections of Brooklyn, police said.

Derron Clarke, of Albany Avenue, was found with a gunshot wound to his neck by officers who were flagged down by a passerby, police said. He was taken to Beth Israel Hospital, where he was declared dead, police said.

The law-enforcement official said the motive for the slaying was not yet known and no arrests have been made.

The two murders in Brooklyn were preceded by the fatal shooting of another man late Saturday, police said.

At about 10:45 p.m. Saturday, 27-year-old Matthew Gray, of Empire Boulevard in Brooklyn, was shot and killed on a residential street in Queens near the Nassau County border, the official said.

Witnesses told police they heard an argument about money followed by the sound of shots, the official said. Mr. Gray was found dead with a gunshot wound to his head in a driveway on 109th Avenue.

The official said video surveillance captured several vehicles fleeing the scene. No arrests have yet been made.

On Sunday in the Bronx, a 27-year-old man was fatally shot at about 4:13 p.m. by an unknown assailant, police said.

William Robles was shot in the head and died at Lincoln Hospital where he was taken after police found him in front of a building on Prospect Avenue in the Morrisania section, police said.

Detectives are investigating what lead to the shooting and no arrests had been made late Sunday, police said.

According to the latest available NYPD crime statistics, as of Oct. 24 there have been 437 murders in New York City in 2010—a 14.4% spike from the same period last year. Overall, major crimes are down 1.17% this year compared with the same period in 2009.