The two were shot at close range about 2:20 a.m. on the 6900 block on Vandike Street in Tacony as they returned from a 7-Eleven around the corner, police said. They were gunned down a block away from the Clark home, where Holloway also had been staying.
Costello said that robbery did not appear to be the motive and that a videotape of the pair at the 7-Eleven showed nothing out of the ordinary as they purchased soda and snacks and left.
Detectives were still seeking a possible motive yesterday.
Medics pronounced Clark dead at the scene. Holloway was in critical condition at Frankford Torresdale Hospital.
Relatives speculated that Clark was killed only because he was a witness to the shooting of Holloway.
Neighborhood residents, who erected shrines at the crime scene and in front of the Clark house, spoke highly of both Clark and Holloway.
They said that Holloway mowed lawns and shoveled walks in the neighborhood and that he paid Clark and other youngsters to help him.
No one could imagine why anyone would hurt the boy, who was to enter the 10th grade at Lincoln High School.
Elizabeth Stier, a block captain from nearby Knorr Street, said that whenever she would put up a poster for a cleanup "he'd be the first kid out there helping out."
She said the sound of gunfire is not unusual in the neighborhood, but the shooting of a teenager was rare.
"Something's got to be done in this city, in every neighborhood," Stier said.
Billy Kinee, 15, said his best friend loved sports - particularly football. As a poster outside Clark's home made clear, he was a Pittsburgh Steelers fan. He was wearing a Phillies cap when he was shot.
"If we'd ever hit a car with a football, he'd be the first one to go over to apologize," Kinee said.
The victim's aunt, Phyllis McBride, said, "That was his routine - a soda every night.
"If someone would walk him, he'd go get it," she said. "Timmy was running home. He was only 15. I hope to God they find who did this," she said.
Robert Boggess, McBride's future son-in-law, said Clark was the sort of kid who would play with younger children.
"He was never in trouble," Boggess said. "Everybody around here loved him. This should have never have happened."
In the first of the two earlier shootings, Costello said, a fight over a "family member being disrespected" sparked gunfire about 7:55 p.m. Thursday at 60th and Spruce Streets in West Philadelphia's Cobbs Creek section.
When the shooting ended, Abdul Allen, 26, was dead, and James Dukes, 24, of the 5800 block of Cedar, had suffered a gunshot wound to the head that left him in critical condition at the Hospital of the University of Pennsylvania, police said.
Costello said police were seeking two, possibly three, gunmen who may have fled in a black Ford Crown Victoria.
In the second incident, brothers Kalhill and Jahmiel Ragin, 27 and 31, respectively, were gunned down about 1:15 a.m. on the 6200 block of Limekiln Pike in East Germantown after an argument with at least one other person in a bar.
During a fistfight outside the bar, a gunman came up and shot both victims, Costello said.
Kahill Regin was killed by a gunshot wound to the head and his brother suffered a shoulder wound, from which he is expected to recover, police said.
The spate of killings followed a one-day lull in homicides in the city.
Costello said there was "no rhyme or reason" why one day could be quiet only to be followed by a "busy night" for the homicide squad.
"It's perilous and it's a grave situation," said Costello, who added that, like others parent raising children in the city, he is concerned about what is happening.
Contact staff writer Joseph Gambardello at 215-854-2153 or email@example.com.