PHILADELPHIA Antonio Flores, 17, was the fourth person killed in a 12-hour period Sunday in Philadelphia, police say.

And detectives are still searching for the 23-year-old who they believe is behind Flores' shooting death inside a friend's apartment in Spring Garden.

Flores was hanging out with friends on the 1600 block of Wallace Street about 3:25 Sunday afternoon when, at some point, one of those friends pulled out a gun and fired once, homicide Capt. James Clark said.

Flores was hit in the head, and the friends in the apartment scattered, Clark said. One stayed behind and called police as Flores lay bleeding on the floor. He was pronounced dead about 20 minutes later at Hahnemann University Hospital.

A warrant has been issued for Sergio Santiago, 23, Clark said. The motive for the shooting is still unclear.

Before Flores' death, detectives fielded three unrelated homicides early Sunday.

The first, at 2:53 a.m., yielded an arrest at the scene. Police took Sophal Siv into custody on the 4900 block of Old York Road in Logan in connection with the shooting death of Hai Luu, a 39-year-old from Cherry Hill. Police said the killing stemmed from an argument the men were having, but did not specify what the argument was about.

Four minutes later, police responded to a report of gunshots near 35th and Wharton Streets in Grays Ferry. They found Rashul Isaacs, 28, outside a nightclub, shot in the neck, an arm, and legs. He was pronounced dead about 10 minutes later at the Hospital of the University of Pennsylvania.

Clark said detectives were seeking the public's help in identifying suspects and added that several people had been outside the bar when the shooting occurred. Police believe someone fired from a car and then drove away, Clark said, but haven't determined a motive in the shooting.

"There were people out there - we need someone to come forward," he said.

At 5:39 a.m., police responded to a report of a person with a weapon on the 6100 block of Hasbrook Avenue in Crescentville. There, they found Luionel Perez, 21, of Fairhill, who had been stabbed in the neck. He was pronounced dead on scene at 5:52 a.m.

Clark said that detectives believed the killing was domestic in nature and were making progress in the case Monday afternoon, but that no arrests had been made.

Anyone with information on any of Sunday's incidents may call the homicide unit at 215-686-3334 or the police tip line at 215-686-TIPS.