YONKERS, N.Y. (CBSNewYork) – Police believe a 15-year-old boy pulled the trigger, shooting and killing a teenage girl in Yonkers who was at the wrong place at the wrong time.

Eighth-grader Jamir Thompson is now wanted for murder in the fatal shooting of Marilyn Cotto Montanez

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“We are considering you armed and dangerous. We are out looking for you as we speak and for the safety of everyone, I urge you to turn yourself in,” Yonkers police commissioner Charles Gardner said.

Police say Thompson is the one who shot and killed the 18-year-old woman last Thursday.

MORE: 18-Year-Old Woman Shot And Killed In Yonkers

She had been walking with her nine-year-old sister when police say she was caught in the crossfire on Morningside and Lake Avenues.

WEB EXTRA – Yonkers Police Detail Their Search For Jamir Thompson:

“The investigation indicates that earlier in the day Thompson had robbed a gold chain from another male – which resulted in this dispute,” Gardner explained.

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Loved ones were left grief stricken, saying the sisters were on their way at the time to get some pizza.

Surveillance video shows Montanez’s little sister after the shooting trying to get some help.

Mayor Mike Spano says he’s spoken with their grieving mother.

“She has tremendous faith and she is coping with this and her family as best as she can,” Spano said.

As for Thompson’s family, police say they are not fully cooperating with the case. Investigators say Thompson lives with his mother in Yonkers and goes to school in Hawthorne.

The 15-year-old is known to police, but the commissioner said authorities couldn’t go into his violent criminal history because of his age.

“We will not rest until Jamir Thompson answers for his reckless and despicable conduct,” Gardner added.

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The commissioner warns anyone hiding Thompson or helping him will also be prosecuted. Police urge anyone with information to come forward immediately.