A police affidavit released Wednesday said 24-year-old Ezequiel Padilla-Villalobos told investigators that his girlfriend asked him to pick up her younger sister Sunday night.
He said he had "thoughts of killing her" while he was driving. Relatives identified the victim as Gabriela Acosta, who was known as Gaby.
Cousin Janeth Acosta told The Denver Post that she went to Broomfield High School and was looking forward to turning 15 and having a quinceanera party.
She said Padilla-Villalobos would sometimes give Gaby rides and was someone the family had trusted.
He is a native of Chihuahua, Mexico and federal immigration officials asked that he be held without bond.