PHOENIX -- A Chandler high school teacher is under police investigation after an 18-year-old male student was stabbed to death by her boyfriend, also a former student, in what appears to be a torrid love triangle, police said.

Chandler police said teacher Tamara Hofmann, 48, called the mother of student Sam Valdivia late Thursday night and asked to speak with Valdivia.

Valdivia's older brother, Julio, said his mother was suspicious of the late-night call.

"It was 11:45 at night, but she told my mom that it had to do with school," Julio Valdivia said.

Sam Valdivia took the call, and shortly thereafter he sneaked off to Hofmann's house.

All the while, Hofmann's boyfriend and former student, Sixto Balbuena, 20, was driving to Hofmann's house on leave from the Navy in California.

Balbuena told police he found his girlfriend naked and Valdivia in his boxer shorts in Hofmann's bedroom around 2:40 a.m. Friday, according to police reports and court documents.

Balbuena said Valdivia apologized to him before Balbuena began kicking, punching and throwing things at the victim, according to a probable cause statement.

Police said Balbuena told them he "wanted to teach (Valdivia) a lesson," and stabbed him in the lower side with a kitchen knife, according to a court document. He also threw Hofmann to the floor, jumped on top of her and demanded to know how long she had been cheating on him, according to the document.

Balbuena called 911 for medical help after the stabbing, police said. He told police he was remorseful after seeing Valdivia lying on the floor and struggling to breathe, police said.

Valdivia was treated by paramedics at the home, but he later died at a hospital.

Valdivia's family believes Hofmann was using him to make Balbuena jealous.

"Why else would she have called at that hour?" said Julio Valdivia.

Chandler police would not confirm the family's theory, only saying their investigation is focused on several different things, including whether or not Hofmann dated other students in the past.

A 2006 Chandler police report shows that police suspected Hofmann of having a sexual relationship with Balbuena, who was 17 at the time.

Both Hofmann and Balbuena denied the relationship and police closed the case.

Hofmann taught Valdivia math at El Dorado High School in Chandler and was also Balbuena's teacher when he attended Marcos de Niza High School in Tempe, police said.

A statement from El Dorado High Schools said school administrators "had no prior knowledge of the alleged inappropriate relationship at the center of the investigation."

El Dorado Principal Ramona Gonzales said she suspended Hofmann after the stabbing incident, and she is currently talking with students to find out more about Hofmann's relationship with Valdivia.

Nevertheless, Gonzales defended her hiring of Hofmann.

"She had 14 years experience, and letters of recommendation," Gonzales said.

Balbuena is in jail in lieu of a $100,000 bond.

Meanwhile, the Valdivia family has set up an account accepting donations to help pay for funeral expenses. Anyone interested can donate at Chase Bank locations under Valdivia's mother's name, Placida Maldonado.

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