Kendra Rae Balentine, 18, faces charges of leaving the scene of a fatal accident.
She was arrested at a gas station after police followed up on a tip made by someone who saw the story on a local media outlet.
Police also recovered the suspected vehicle, a 2000 Chevy Cavalier, two-door, silver in color with a temporary plate. Police say the car shows signs of a "pedestrian accident."
At 9:48 p.m. on Monday, Longmont Police responded to a fight on the 2200 block of Main Street. Police could not locate all the fighting parties involved, so they left the scene.
A few minutes later, officers returned to the scene when they heard about another fight in the same area.
Police say as Jason Grimmer ran away from the encounter, he was hit by a vehicle traveling southbound on Main Street.
Balentine is related to some of the people involved in the fight, according to Commander Jeff Satur with the Longmont Police Department.
Police say she witnessed the fight but was not part of it. Balentine denies any involvement in the incident.