Premila Lal jumped out of a closet as a harmless joke to surprise a family friend, her father says. But the noise startled the friend, who grabbed a gun and shot her.
The incident happened Friday night in the city about an hour north of Denver.
Just before 8 p.m. Premila Lal showed up at a house her family once called home. They had since moved, and were preparing to sell the place. In the meantime, Nerrek Galley, a 21-year-old friend of the family, was staying at the house.
Lal and her 12-year-old cousin apparently came to visit Galley. Lal's 15-year-old brother was also at the home, hanging out with Galley.
The girl's father, Praveen Lal, said in interviews with local media that his daughter entered the home unbeknownst to Galley.
Authorities say the father's description of what happened next -- involving the closet, and Galley's gun -- is consistent with their investigation.
"All the witnesses had consistent statements," Commander Jeffrey Satur of the Longmont Police Department told CNN Sunday.
A neighbor told CNN's affilate KVDR that she saw the group rush out of the home, screaming. Lal was admitted to a hospital and was pronounced dead about an hour later.
"We lost a daughter, so we don't want anybody else to lose their son, you know, especially when it was accident," Praveen Lal told Denver TV station KCNC.
Galley has been charged with reckless endangerment, child abuse and providing a gun to a juvenile because Lal's 15-year-old brother had access to the gun, police say.
So far Galley has not been charged in Lal's killing.
It was not immediately clear whether he has spoken with an attorney. No public statement has been made on his behalf.
Police said they do not suspect the involvement of alcohol or drugs.
The teen's death was "unintended and extremely tragic," police said in a statement.
Lal was honored at a weekend cross country meet at her alma mater, KVDR reported.
Her family described her as an athlete, popular girl and strong student.
Longmont is a former farming community at the foothills of the Rockies. More recently, the community of just under 90,000 has developed high-tech jobs.
The town was already stunned by a murder-suicide a day earlier. An 80-year-old man shot his 72-year-old wife before killing himself, police say.
Before last week, Satur said, he can't remember when the last shooting was. "It's been a tough weekend."
CNN's John Branch contributed to this report.