Police on Wednesday night had two teenage boys in custody in the shooting death last week of Kristian Marche, the 18-year-old track standout from Imhotep Charter High School who was gunned down after he confronted a pair of teens trying to break into his family's house.

Taron Small, 16, and Byron Vinson, 15, were trying to break into Marche's home in the city's West Oak Lane section on the night of Aug. 13 when they were confronted by Marche, who was set to begin at Pennsylvania State University this fall, police said.

Homicide Unit Lt. Norman Davenport said Marche heard people attempting to break in through the back of the house on the 1800 block of East Pastorius Street and went outside. After twice encountering the teens, Marche began walking away when Small shot him in the back of the head, police said.

"There was not a struggle, there was not a fight," Davenport said. "This was clearly a case of murder."

Small was arrested Wednesday evening in the 2500 block of Carlisle Street in North Philadelphia, police said. Vinson had been arrested earlier in the day.

Detectives believed the teens were acquainted with one another, Davenport said, but investigators were still seeking to determine the nature of their relationship — and why the younger boys would have targeted Marche's house.

The lieutenant said the case was another tragic incident involving young people in the city. As of Monday, according to police statistics, at least 102 people age 18 or younger had been shot in Philadelphia this year.

"This case really hit a lot of detectives hard," Davenport said, noting that the killing was captured on surveillance video. "We need to come together as a community and decide what can we do to make a change."

> > READ MORE: Coaches, friends mourn slain Imhotep track star headed to Penn State

Vinson and Small were each expected to face counts including murder and conspiracy.

According to Davenport, Marche had been at home with a family member when he heard noises and saw someone jump off a back porch. Marche ran outside and confronted Vinson and Small, Davenport said, then went back inside for what Davenport called "a short period of time."

Marche went back outside and again encountered the teens, Davenport said. As Marche began walking toward his home after the second interaction, Davenport said, Small — who was behind Marche — pulled a gun and fired.

Marche was taken to Einstein Medical Center, where he was declared dead around 6:20 p.m. on Aug. 14 — the same day he had planned to head to Penn State to start his freshman year.

Through the Police Department, family members declined to comment Wednesday. Friends and coaches said Marche, a sprinter, was one of the state's fastest high school runners . He had also played football at Imhotep but secured a partial track-and-field scholarship to go to Penn State.

His personal track coach, Diamond Woolford, said Wednesday that he was "still trying to grasp it all" after hearing the latest police version of events. He said Marche had a personality "where he could literally turn a complete stranger … into a friend within a matter of seconds" and that he had not heard Marche discuss trouble with either of the murder suspects.

"It just sucks," Woolford said. "He was about to leave in a few hours to go up" to Penn State."

Staff writer Robert Moran contributed to this article.