HOUSTON -

A 19-year-old man has been charged with capital murder after police say he confessed to killing his girlfriend.

Court documents filed Thursday detail the allegations against Luan Vu, saying he kidnapped Vy Ngoc Bao-Pham, struck her with a branch and choked her.

Bao-Pham, 17, had been missing since Monday and police issued an Amber Alert because it was believed she had been abducted. The Chavez High School student was last seen in the 8200 block of Broadway in southeast Houston early Monday morning. She left her family's apartment and was walking to visit a friend at a complex next door, but never made it.

Shortly after she disappeared Monday, her family received a ransom text message demanding $50,000. The text went on to say that if the money was not paid in two weeks, then Pham would die.

Police told Local 2 they were able to trace that text message back to Pham's boyfriend, Luan Vu. They brought Vu in for questioning and say he began to confess, and even offered up a motive.

"He was very open about what he had done, why he did it and where he put her," said Detective Fil Waters with HPD Homicide. "He was infatuated with Vy. They had a pretty passionate relationship and she had made some indications to him she had acquired a new boyfriend. His response to that was to assault her and take her to a place where he eventually killed her."

Waters said Vu lead detectives to the 4700 block of Broadway on Wednesday and told them that's where they would find Pham's body. The dive team entered the water of Sims Bayou just after 6 p.m. and recovered the body a short time later.

"There are some indications on the body how the death occurred but we're not going to be able to discuss it.  That's something an autopsy's going to have to do," Waters said.

Detectives said Vu told them he wanted to test their investigating skills and see how quickly they could solve a crime.

"He wanted to see if we were as good as he heard we were," Waters said.

Vu is described by police as an intelligent video gamer.  He's been in the country for five years and lives with relatives. They were unsure if he had a job or went to school.

Bao-Pham was a student at Chavez High School in southeast Houston.  Grief counselors were on campus Thursday to help students deal with her death.