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LONGMONT, Colo. — A 15-year-old has been arrested after a fatal stabbing at a Longmont apartment complex on Saturday, police said.

The Longmont Police Department responded to the Shores Apartments at 2450 Airport Road just after 6 p.m.

When officers arrived, they found a female victim had been stabbed and she was taken to a hospital where she later died.

The suspect fled the scene before officers arrived, but was arrested about 1 a.m. Sunday in Lakewood, according to police.

A father said he was washing dishes when he heard the commotion outside his front door.

“I️ heard a thud, thud, thud I️ opened up my living room door I️ looked out here and I️ saw a girl laying right here on the floor,” Michael Brown said.

The woman was bleeding badly, but was still alive when emergency crews arrived.

“She wasn’t responding. I️ yelled, are you OK? And I️ screamed for help for someone to call 911,” Brown said.

Brown, said then, “A man with a motorcycle jacket, motorcycle helmet gloves and a knife in his hands was coming down the stairs and he saw me. I️ backed up from her and when he saw me backing up I️ ran further, because he started coming after me faster.”

Brown then went for his gun and to make sure his child was protected.

Dani O’Connell, who lives at the complex said “the fact that it was a 15-year-old kid that did it is just crazy to me.”

“I️ find that so devastating for the murder victim and her family, for the kid,” said Jeanette, who did not use his full name. “What brings us to this point as a community? What gets a 15-year-old kid to do that anywhere. But it’s so rare here it hurts more.”

Investigators say the teen knew the victim who police describe as a young adult.

Brown said she was his neighbor and lived on the floor above his unit.

The Boulder County Medical Examiner has not released the victim’s name.

The teenager is being held in the Boulder County Juvenile detention Center where he is charged with first-degree murder.