PHILADELPHIA (CBS) — A 16-year-old boy was gunned down outside a Philadelphia convenience store. Police have identified the teen’s killer but that’s as far as the case has gone.

For this week’s CBS3 Mysteries, it’s not a question of who did it or even what happened. The mystery is where is the killer?

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“He would’ve turned 18 on April 19,” Brittany Brunson said.

Brunson thinks of all the things that could have been — the prom is just days away and then graduation in a few weeks. Her son, Kahlief Myrick, is no longer here for any of it.

Another Mother’s Day will slip by.

His plot at the cemetery still awaits a gravestone. He was just 16 years old when he was murdered last winter. Brunson admits she doesn’t like coming here.

“It’s just a pain. It’s just like one of the scars you can’t see, but it’s never going away,” she said.

Khalief was inside a Southwest Philadelphia 7-Eleven very early on the morning of February 18, 2021. Investigators say he had words with Donald Whitingham inside the store.

Brunson says her son had a tough exterior.

Police say Whitingham waited for Khalief outside the store where he fatally shot him in cold blood.

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Brunson says she wants the chance to confront Whitingham. She has questions.

“He was a caring young man just trying in the inner city. He had to have that hard exterior coming up what he came up around. I’m trying to tell you if you would’ve known him you wouldn’t have wanted to do that to him,” Brunson said.

Whitingham’s face is plastered on most wanted fugitive posters. The U.S. Marshals are searching for him. His whereabouts are a mystery.

The loss of Khalief leaves so many emotions.

“It almost feels like guilt on my part, because I feel like I wasn’t there to protect him. It’s just a guilt, it’s like a failure I feel,” Brunson said.

Always the explorer, Brunson said her son loved music and fashion. He was energetic and charismatic.

She says she warned him about those Philadelphia streets.

She had these words for Whitingham.

“I just want you to know that my heart, I’m forever broken, forever frightened for my other son. Forever, until the day I leave this earth, I will have a broken heart. That was my first baby. That’s what I want you to know, that my heart is broken,” Brunson said.

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If you know where Whitingham is, police are offering a reward. Call 215-686-TIPS with any information.